diff mbox

libio: Implement internal function __libc_readline_unlocked

Message ID ab9faefa-7875-4380-fca0-87767bf271f0@redhat.com
State New, archived
Headers show

Commit Message

Florian Weimer Sept. 2, 2017, 7:10 p.m. UTC
On 09/01/2017 08:00 PM, Florian Weimer wrote:
> This should use __underflow instead of __uflow, so that the need for the
> special handling for '\n' goes away.

Here is the version based on __underflow.  I changed the way the
__ftello64 and __fseeko64 aliases are handled because the other patch
needs to call __fseko64 from a DSO, so the hidden symbol no longer works.

Thanks,
Florian

Comments

Florian Weimer Sept. 14, 2017, 6:56 a.m. UTC | #1
On 09/02/2017 09:10 PM, Florian Weimer wrote:
> On 09/01/2017 08:00 PM, Florian Weimer wrote:
>> This should use __underflow instead of __uflow, so that the need for the
>> special handling for '\n' goes away.
> 
> Here is the version based on __underflow.  I changed the way the
> __ftello64 and __fseeko64 aliases are handled because the other patch
> needs to call __fseko64 from a DSO, so the hidden symbol no longer works.

Ping?

  <https://sourceware.org/ml/libc-alpha/2017-09/msg00121.html>

Thanks,
Florian
Florian Weimer Oct. 5, 2017, 4:44 p.m. UTC | #2
On 09/14/2017 08:56 AM, Florian Weimer wrote:
> On 09/02/2017 09:10 PM, Florian Weimer wrote:
>> On 09/01/2017 08:00 PM, Florian Weimer wrote:
>>> This should use __underflow instead of __uflow, so that the need for the
>>> special handling for '\n' goes away.
>>
>> Here is the version based on __underflow.  I changed the way the
>> __ftello64 and __fseeko64 aliases are handled because the other patch
>> needs to call __fseko64 from a DSO, so the hidden symbol no longer works.

>    <https://sourceware.org/ml/libc-alpha/2017-09/msg00121.html>

Ping?

Thanks,
Florian
Jonathan Nieder Nov. 22, 2017, 5:50 a.m. UTC | #3
Florian Weimer wrote:

> --- a/libio/ftello.c
> +++ b/libio/ftello.c
> @@ -61,4 +61,6 @@ weak_alias (__ftello, ftello)
> 
>  #ifdef __OFF_T_MATCHES_OFF64_T
>  weak_alias (__ftello, ftello64)
> +strong_alias (__ftello, __ftello64)

What does this part do?

[...]
> +++ b/libio/readline.c
> @@ -0,0 +1,169 @@
[...]
> +/* Slow path for reading the line.  Called with no data in the stream
> +   read buffer.  Write data to [BUFFER, BUFFER_END).  */
> +static ssize_t
> +readline_slow (_IO_FILE *fp, char *buffer, char *buffer_end)
> +{
> +  char *start = buffer;
> +
> +  while (buffer < buffer_end)
> +    {
> +      if (__underflow (fp) == EOF)
> +        {
> +          if (_IO_ferror_unlocked (fp))
> +            /* If the EOF was caused by a read error, report it.  */
> +            return fail_no_erange ();

Is readline_slow's caller responsible for ensuring !ferror(fp) before
the __underflow call?  Otherwise it's possible for this to return -1
without setting errno.

> +          *buffer = '\0';
> +          /* Do not include the NULL terminator.  */

nit: s/NULL/NUL/ or s/NULL/null/ (NULL is a pointer, NUL an ascii code
point).

> +          return buffer - start;
> +        }
[...]
> +      char *pnl = memchr (readptr, '\n', readlen);
> +      if (pnl != NULL)
> +        {
> +          /* We found the terminator.  */
> +          size_t line_length = pnl - readptr;
> +          if (line_length + 2 > buffer_end - buffer)
> +            /* Not enough room in the caller-supplied buffer.  */
> +            break;
> +          memcpy (buffer, readptr, line_length + 1);
> +          buffer[line_length + 1] = '\0';
> +          fp->_IO_read_ptr = pnl + 1;
> +          /* Do not include the NULL terminator.  */

Same nit about \0 vs NULL.

> +          return buffer - start + line_length + 1;
> +        }
[...]
> +ssize_t
> +__libc_readline_unlocked (_IO_FILE *fp, char *buffer, size_t buffer_length)
> +{
> +  char *buffer_end = buffer + buffer_length;
> +
> +  /* Orient the stream.  */
> +  if (__builtin_expect (fp->_mode, -1) == 0)
> +    _IO_fwide (fp, -1);
> +
> +  /* Fast path: The line terminator is found in the buffer.  */
> +  char *readptr = fp->_IO_read_ptr;
> +  ssize_t readlen = fp->_IO_read_end - readptr;
> +  off64_t start_offset;         /* File offset before reading anything.  */
> +  if (readlen > 0)
> +    {

optional: could handle the !readlen case first to avoid leaving the
reader in suspense.

> +      char *pnl = memchr (readptr, '\n', readlen);
[...]
> +      buffer += readlen;
> +      /* The original length is invalid after this point.  Use
> +         buffer_end instead.  */
> +#pragma GCC poison buffer_length

Neat!  I hadn't seen this trick before.

[...]
> +  /* Slow path: Read more data from the underlying file.  We need to
> +     restore the file pointer if the buffer is too small.  First,
> +     check if the __ftello64 call above failed.  */
> +  if (start_offset < 0)
> +    return fail_no_erange ();
> +
> +  ssize_t result = readline_slow (fp, buffer, buffer_end);
> +  if (result < 0)
> +    {
> +      if (errno == ERANGE)
> +        {
> +          /* Restore the file pointer so that the caller may read the
> +             same line again.  */
> +          if (__fseeko64 (fp, start_offset, SEEK_SET) < 0)
> +            return fail_no_erange ();

What happens if the file is unseekable?  Is this function not meant to
be used for such files, or is this seeking to reset the file pointer a
best-effort kind of thing?

[...]
> +  return readlen + result;
> +}

Looks good.  The test also looks good.

The only part that worries me is the value of errno if ferror(fp) was
already true.  The rest of my observations are cosmetic.

Thanks and hope that helps,
Jonathan
Andreas Schwab Nov. 22, 2017, 10:11 a.m. UTC | #4
On Nov 21 2017, Jonathan Nieder <jrnieder@gmail.com> wrote:

> nit: s/NULL/NUL/ or s/NULL/null/ (NULL is a pointer, NUL an ascii code
> point).

NULL is the official name of the Unicode character U+0000, though most
presentations use the abbreviation.

Andreas.
Florian Weimer July 6, 2018, 12:31 p.m. UTC | #5
On 11/22/2017 06:50 AM, Jonathan Nieder wrote:
> Florian Weimer wrote:
> 
>> --- a/libio/ftello.c
>> +++ b/libio/ftello.c
>> @@ -61,4 +61,6 @@ weak_alias (__ftello, ftello)
>>
>>   #ifdef __OFF_T_MATCHES_OFF64_T
>>   weak_alias (__ftello, ftello64)
>> +strong_alias (__ftello, __ftello64)
> 
> What does this part do?

If off_t and off64_t are the same type, __ftello and __ftello64 are the 
same function (at the same address).

> [...]
>> +++ b/libio/readline.c
>> @@ -0,0 +1,169 @@
> [...]
>> +/* Slow path for reading the line.  Called with no data in the stream
>> +   read buffer.  Write data to [BUFFER, BUFFER_END).  */
>> +static ssize_t
>> +readline_slow (_IO_FILE *fp, char *buffer, char *buffer_end)
>> +{
>> +  char *start = buffer;
>> +
>> +  while (buffer < buffer_end)
>> +    {
>> +      if (__underflow (fp) == EOF)
>> +        {
>> +          if (_IO_ferror_unlocked (fp))
>> +            /* If the EOF was caused by a read error, report it.  */
>> +            return fail_no_erange ();
> 
> Is readline_slow's caller responsible for ensuring !ferror(fp) before
> the __underflow call?  Otherwise it's possible for this to return -1
> without setting errno.

The caller should not call the function if the stream is in an error 
condition I added a comment to the header file.

> 
>> +          *buffer = '\0';
>> +          /* Do not include the NULL terminator.  */
> 
> nit: s/NULL/NUL/ or s/NULL/null/ (NULL is a pointer, NUL an ascii code
> point).

Thanks, fixed.

> [...]
>> +  /* Slow path: Read more data from the underlying file.  We need to
>> +     restore the file pointer if the buffer is too small.  First,
>> +     check if the __ftello64 call above failed.  */
>> +  if (start_offset < 0)
>> +    return fail_no_erange ();
>> +
>> +  ssize_t result = readline_slow (fp, buffer, buffer_end);
>> +  if (result < 0)
>> +    {
>> +      if (errno == ERANGE)
>> +        {
>> +          /* Restore the file pointer so that the caller may read the
>> +             same line again.  */
>> +          if (__fseeko64 (fp, start_offset, SEEK_SET) < 0)
>> +            return fail_no_erange ();
> 
> What happens if the file is unseekable?  Is this function not meant to
> be used for such files, or is this seeking to reset the file pointer a
> best-effort kind of thing?

The function is not meant to be used for such files.

I've rebased the patch and fixed the internal alias generation logic.

Carlos, is this and the fix for bug 18991 (which has already been 
reviewed by DJ) still acceptable during the freeze?

Thanks,
Florian
diff mbox

Patch

libio: Implement internal function __libc_readline_unlocked

This is a variant of fgets which fails with ERANGE if the
buffer is too small, and the buffer length is given as an
argument of type size_t.

This function will be useful for implementing NSS file reading
operations.  Compared to a direct implementation using the public API,
it avoids an lseek system call in case the line terminator can be
found in the internal read buffer.

2017-09-02  Florian Weimer  <fweimer@redhat.com>

	* include/stdio.h (__libc_readline_unlocked): Declare.
	(__ftello64, __fseeko64): Declare aliases.
	* libio/readline.c: New file.
	* libio/tst-readline.c: Likewise.
	(routines): Add readline.
	(tests-internal): Add tst-readlime.
	* libio/Versions (GLIBC_PRIVATE): Export __fseeko64, __ftello64,
	__libc_readline_unlocked.
	* libio/fseeko.c (__fseeko): Rename from fseeko.
	(fseeko): Add alias.
	[__OFF_T_MATCHES_OFF64_T] (fseeko64, __fseeko64): Likewise.
	* libio/fseeko64.c (__fseeko64): Rename from fseeko64.
	(fseeko64): Add alias.
	* libio/ftello.c [__OFF_T_MATCHES_OFF64_T] (__ftello64): Add alias.
	* libio/ftello64.c (__ftello64): Rename from ftello64.
	(ftello64): Add alias.

diff --git a/include/stdio.h b/include/stdio.h
index 509447c528..50b4191279 100644
--- a/include/stdio.h
+++ b/include/stdio.h
@@ -122,6 +122,18 @@  extern int __fxprintf (FILE *__fp, const char *__fmt, ...)
 extern int __fxprintf_nocancel (FILE *__fp, const char *__fmt, ...)
      __attribute__ ((__format__ (__printf__, 2, 3)));
 
+/* Read the next line from FP into BUFFER, of LENGTH bytes.  LINE will
+   include the line terminator and a NUL terminator.  On success,
+   return the length of the line, including the line terminator, but
+   excluding the NUL termintor.  On EOF, return zero and write a NUL
+   terminator.  On error, return -1 and set errno.  If the total byte
+   count (line and both terminators) exceeds LENGTH, return -1 and set
+   errno to ERANGE (but do not mark the stream as failed).
+
+   The behavior is undefined if FP is not seekable.  */
+ssize_t __libc_readline_unlocked (_IO_FILE *fp, char *buffer, size_t length);
+libc_hidden_proto (__libc_readline_unlocked);
+
 extern const char *const _sys_errlist_internal[] attribute_hidden;
 extern int _sys_nerr_internal attribute_hidden;
 
@@ -161,6 +173,10 @@  libc_hidden_proto (fwrite)
 libc_hidden_proto (fseek)
 extern __typeof (ftello) __ftello;
 libc_hidden_proto (__ftello)
+extern __typeof (fseeko64) __fseeko64;
+libc_hidden_proto (__fseeko64)
+extern __typeof (ftello64) __ftello64;
+libc_hidden_proto (__ftello64)
 libc_hidden_proto (fflush)
 libc_hidden_proto (fflush_unlocked)
 extern __typeof (fflush_unlocked) __fflush_unlocked;
diff --git a/libio/Makefile b/libio/Makefile
index 9d09bd8b6a..85da7dc42f 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -47,7 +47,7 @@  routines	:=							      \
 	__fbufsize __freading __fwriting __freadable __fwritable __flbf	      \
 	__fpurge __fpending __fsetlocking				      \
 									      \
-	libc_fatal fmemopen oldfmemopen vtables
+	libc_fatal fmemopen oldfmemopen vtables readline
 
 tests = tst_swprintf tst_wprintf tst_swscanf tst_wscanf tst_getwc tst_putwc   \
 	tst_wprintf2 tst-widetext test-fmemopen tst-ext tst-ext2 \
@@ -63,6 +63,9 @@  tests = tst_swprintf tst_wprintf tst_swscanf tst_wscanf tst_getwc tst_putwc   \
 	tst-setvbuf1 tst-popen1 tst-fgetwc bug-wsetpos tst-fseek \
 	tst-fwrite-error tst-ftell-partial-wide tst-ftell-active-handler \
 	tst-ftell-append tst-fputws
+
+tests-internal = tst-readline
+
 ifeq (yes,$(build-shared))
 # Add test-fopenloc only if shared library is enabled since it depends on
 # shared localedata objects.
diff --git a/libio/Versions b/libio/Versions
index 77123347e3..acb896afa9 100644
--- a/libio/Versions
+++ b/libio/Versions
@@ -158,5 +158,9 @@  libc {
 
     # Used by NPTL
     _IO_enable_locks;
+
+    __fseeko64;
+    __ftello64;
+    __libc_readline_unlocked;
   }
 }
diff --git a/libio/fseeko.c b/libio/fseeko.c
index b77d4be18f..606a30366f 100644
--- a/libio/fseeko.c
+++ b/libio/fseeko.c
@@ -28,7 +28,7 @@ 
 #include "stdio.h"
 
 int
-fseeko (_IO_FILE *fp, off_t offset, int whence)
+__fseeko (_IO_FILE *fp, off_t offset, int whence)
 {
   int result;
   CHECK_FILE (fp, -1);
@@ -38,6 +38,10 @@  fseeko (_IO_FILE *fp, off_t offset, int whence)
   return result;
 }
 
+weak_alias (__fseeko, fseeko)
+
 #ifdef __OFF_T_MATCHES_OFF64_T
-weak_alias (fseeko, fseeko64)
+weak_alias (__fseeko, fseeko64)
+strong_alias (__fseeko, __fseeko64)
+libc_hidden_ver (__fseeko, __fseeko64)
 #endif
diff --git a/libio/fseeko64.c b/libio/fseeko64.c
index 12221d585e..fd612f4ed8 100644
--- a/libio/fseeko64.c
+++ b/libio/fseeko64.c
@@ -32,7 +32,7 @@ 
 #ifndef __OFF_T_MATCHES_OFF64_T
 
 int
-fseeko64 (_IO_FILE *fp, __off64_t offset, int whence)
+__fseeko64 (_IO_FILE *fp, __off64_t offset, int whence)
 {
   int result;
   CHECK_FILE (fp, -1);
@@ -42,4 +42,8 @@  fseeko64 (_IO_FILE *fp, __off64_t offset, int whence)
   return result;
 }
 
+libc_hidden_def (__fseeko64)
+
+weak_alias (__fseeko64, fseeko64)
+
 #endif
diff --git a/libio/ftello.c b/libio/ftello.c
index 9791e835c6..a11c54d63c 100644
--- a/libio/ftello.c
+++ b/libio/ftello.c
@@ -61,4 +61,6 @@  weak_alias (__ftello, ftello)
 
 #ifdef __OFF_T_MATCHES_OFF64_T
 weak_alias (__ftello, ftello64)
+strong_alias (__ftello, __ftello64)
+libc_hidden_ver (__ftello, __ftello64)
 #endif
diff --git a/libio/ftello64.c b/libio/ftello64.c
index 15518af7a6..afa0703f2e 100644
--- a/libio/ftello64.c
+++ b/libio/ftello64.c
@@ -32,7 +32,7 @@ 
 #ifndef __OFF_T_MATCHES_OFF64_T
 
 off64_t
-ftello64 (_IO_FILE *fp)
+__ftello64 (_IO_FILE *fp)
 {
   _IO_off64_t pos;
   CHECK_FILE (fp, -1L);
@@ -53,4 +53,8 @@  ftello64 (_IO_FILE *fp)
   return pos;
 }
 
+libc_hidden_def (__ftello64)
+
+weak_alias (__ftello64, ftello64)
+
 #endif
diff --git a/libio/readline.c b/libio/readline.c
new file mode 100644
index 0000000000..0d46844c1a
--- /dev/null
+++ b/libio/readline.c
@@ -0,0 +1,169 @@ 
+/* fgets with ERANGE error reporting and size_t buffer length.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <assert.h>
+#include <errno.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "libioP.h"
+
+/* Return -1 and set errno to EINVAL if it is ERANGE.  */
+static ssize_t
+fail_no_erange (void)
+{
+  if (errno == ERANGE)
+    __set_errno (EINVAL);
+  return -1;
+}
+
+/* Slow path for reading the line.  Called with no data in the stream
+   read buffer.  Write data to [BUFFER, BUFFER_END).  */
+static ssize_t
+readline_slow (_IO_FILE *fp, char *buffer, char *buffer_end)
+{
+  char *start = buffer;
+
+  while (buffer < buffer_end)
+    {
+      if (__underflow (fp) == EOF)
+        {
+          if (_IO_ferror_unlocked (fp))
+            /* If the EOF was caused by a read error, report it.  */
+            return fail_no_erange ();
+          *buffer = '\0';
+          /* Do not include the NULL terminator.  */
+          return buffer - start;
+        }
+
+      /* __underflow has filled the buffer.  */
+      char *readptr = fp->_IO_read_ptr;
+      ssize_t readlen = fp->_IO_read_end - readptr;
+      assert (readlen > 0);
+      char *pnl = memchr (readptr, '\n', readlen);
+      if (pnl != NULL)
+        {
+          /* We found the terminator.  */
+          size_t line_length = pnl - readptr;
+          if (line_length + 2 > buffer_end - buffer)
+            /* Not enough room in the caller-supplied buffer.  */
+            break;
+          memcpy (buffer, readptr, line_length + 1);
+          buffer[line_length + 1] = '\0';
+          fp->_IO_read_ptr = pnl + 1;
+          /* Do not include the NULL terminator.  */
+          return buffer - start + line_length + 1;
+        }
+
+      if (readlen >= buffer_end - buffer)
+        /* Not enough room in the caller-supplied buffer.  */
+        break;
+
+      /* Save and consume the stream buffer.  */
+      memcpy (buffer, readptr, readlen);
+      fp->_IO_read_ptr = fp->_IO_read_end;
+      buffer += readlen;
+    }
+
+  /* The line does not fit into the buffer.  */
+  __set_errno (ERANGE);
+  return -1;
+}
+
+ssize_t
+__libc_readline_unlocked (_IO_FILE *fp, char *buffer, size_t buffer_length)
+{
+  char *buffer_end = buffer + buffer_length;
+
+  /* Orient the stream.  */
+  if (__builtin_expect (fp->_mode, -1) == 0)
+    _IO_fwide (fp, -1);
+
+  /* Fast path: The line terminator is found in the buffer.  */
+  char *readptr = fp->_IO_read_ptr;
+  ssize_t readlen = fp->_IO_read_end - readptr;
+  off64_t start_offset;         /* File offset before reading anything.  */
+  if (readlen > 0)
+    {
+      char *pnl = memchr (readptr, '\n', readlen);
+      if (pnl != NULL)
+        {
+          size_t line_length = pnl - readptr;
+          /* Account for line and NULL terminators.  */
+          if (line_length + 2 > buffer_length)
+            {
+              __set_errno (ERANGE);
+              return -1;
+            }
+          memcpy (buffer, readptr, line_length + 1);
+          buffer[line_length + 1] = '\0';
+          /* Consume the entire line.  */
+          fp->_IO_read_ptr = pnl + 1;
+          return line_length + 1;
+        }
+
+      /* If the buffer does not have enough space for what is pending
+         in the stream (plus a NUL terminator), the buffer is too
+         small.  */
+      if (readlen + 1 > buffer_length)
+        {
+          __set_errno (ERANGE);
+          return -1;
+        }
+
+      /* End of line not found.  We need all the buffered data.  Fall
+         through to the slow path.  */
+      memcpy (buffer, readptr, readlen);
+      buffer += readlen;
+      /* The original length is invalid after this point.  Use
+         buffer_end instead.  */
+#pragma GCC poison buffer_length
+      /* Read the old offset before updating the read pointer.  */
+      start_offset = __ftello64 (fp);
+      fp->_IO_read_ptr = fp->_IO_read_end;
+    }
+  else
+    {
+      readlen = 0;
+      start_offset = __ftello64 (fp);
+    }
+
+  /* Slow path: Read more data from the underlying file.  We need to
+     restore the file pointer if the buffer is too small.  First,
+     check if the __ftello64 call above failed.  */
+  if (start_offset < 0)
+    return fail_no_erange ();
+
+  ssize_t result = readline_slow (fp, buffer, buffer_end);
+  if (result < 0)
+    {
+      if (errno == ERANGE)
+        {
+          /* Restore the file pointer so that the caller may read the
+             same line again.  */
+          if (__fseeko64 (fp, start_offset, SEEK_SET) < 0)
+            return fail_no_erange ();
+          __set_errno (ERANGE);
+        }
+      /* Do not restore the file position on other errors; it is
+         likely that the __fseeko64 call would fail, too.  */
+      return -1;
+    }
+  return readlen + result;
+}
+libc_hidden_def (__libc_readline_unlocked)
diff --git a/libio/tst-readline.c b/libio/tst-readline.c
new file mode 100644
index 0000000000..79ed6ea699
--- /dev/null
+++ b/libio/tst-readline.c
@@ -0,0 +1,235 @@ 
+/* Test the __libc_readline_unlocked function.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+/* Exercise __libc_readline_unlocked with various combinations of line
+   lengths, stdio buffer sizes, and line read buffer sizes.  */
+
+#include <errno.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <support/check.h>
+#include <support/test-driver.h>
+#include <support/support.h>
+#include <support/xstdio.h>
+#include <support/xmemstream.h>
+#include <support/temp_file.h>
+#include <support/xunistd.h>
+
+enum
+  {
+    maximum_line_length = 7,
+    number_of_lines = 3,
+  };
+
+/* -1: Do not set buffer size.  0: unbuffered.  Otherwise, use this as
+   the size of the buffer.  */
+static int buffer_size;
+
+/* These size of the buffer used for reading.  Must be at least 2.  */
+static int read_size;
+
+/* If a read files with ERANGE, increase the buffer size by this
+   amount.  Must be positive.  */
+static int read_size_increment;
+
+/* If non-zero, do not reset the read size after an ERANGE error.  */
+static int read_size_preserve;
+
+/* If non-zero, no '\n' at the end of the file.  */
+static int no_newline_at_eof;
+
+/* Length of the line, or -1 if the line is not present.  */
+static int line_lengths[number_of_lines];
+
+/* The name of the test file.  */
+static char *test_file_path;
+
+/* The contents of the test file.  */
+static char expected_contents[(maximum_line_length + 2) * number_of_lines + 1];
+static size_t expected_length;
+
+/* Returns a random byte which is not zero or the line terminator.  */
+static char
+random_char (void)
+{
+  static unsigned int rand_state = 1;
+  while (true)
+    {
+      char result = rand_r (&rand_state) >> 16;
+      if (result != 0 && result != '\n')
+        return result;
+    }
+}
+
+/* Create the test file.  */
+static void
+prepare (int argc, char **argv)
+{
+  int fd = create_temp_file ("tst-readline-", &test_file_path);
+  TEST_VERIFY_EXIT (fd >= 0);
+  xclose (fd);
+}
+
+/* Prepare the test file.  Return false if the test parameters are
+   incongruent and the test should be skipped.  */
+static bool
+write_test_file (void)
+{
+  expected_length = 0;
+  char *p = expected_contents;
+  for (int lineno = 0; lineno < number_of_lines; ++lineno)
+    for (int i = 0; i < line_lengths[lineno]; ++i)
+      *p++ = random_char ();
+  expected_length = p - &expected_contents[0];
+  if (no_newline_at_eof)
+    {
+      if (expected_length == 0)
+        return false;
+      --expected_length;
+      --p;
+    }
+  if (test_verbose > 0)
+    {
+      printf ("info: writing test file of %zu bytes:\n", expected_length);
+      for (int i = 0; i < number_of_lines; ++i)
+        printf (" line %d: %d\n", i, line_lengths[i]);
+      if (no_newline_at_eof)
+        puts ("  (no newline at EOF)");
+    }
+  TEST_VERIFY_EXIT (expected_length < sizeof (expected_contents));
+  *p++ = '\0';
+  support_write_file_string (test_file_path, expected_contents);
+  return true;
+}
+
+/* Run a single test (a combination of a test file and read
+   parameters).  */
+static void
+run_test (void)
+{
+  TEST_VERIFY_EXIT (read_size_increment > 0);
+  if (test_verbose > 0)
+    {
+      printf ("info: running test: buffer_size=%d read_size=%d\n"
+              "  read_size_increment=%d read_size_preserve=%d\n",
+              buffer_size, read_size, read_size_increment, read_size_preserve);
+    }
+
+  struct xmemstream result;
+  xopen_memstream (&result);
+
+  FILE *fp = xfopen (test_file_path, "rce");
+  char *fp_buffer = NULL;
+  if (buffer_size == 0)
+    TEST_VERIFY_EXIT (setvbuf (fp, NULL, _IONBF, 0) == 0);
+  if (buffer_size > 0)
+    {
+      fp_buffer = xmalloc (buffer_size);
+      TEST_VERIFY_EXIT (setvbuf (fp, fp_buffer, _IOFBF, buffer_size) == 0);
+    }
+
+  char *line_buffer = xmalloc (read_size);
+  size_t line_buffer_size = read_size;
+
+  while (true)
+    {
+      ssize_t ret = __libc_readline_unlocked
+        (fp, line_buffer, line_buffer_size);
+      if (ret < 0)
+        {
+          TEST_VERIFY (ret == -1);
+          if (errno != ERANGE)
+            FAIL_EXIT1 ("__libc_readline_unlocked: %m");
+          line_buffer_size += read_size_increment;
+          free (line_buffer);
+          line_buffer = xmalloc (line_buffer_size);
+          /* Try reading this line again.  */
+        }
+      else if (ret == 0)
+        break;
+      else
+        {
+          /* A line has been read.  Save it.  */
+          TEST_VERIFY (ret == strlen (line_buffer));
+          const char *pnl = strchr (line_buffer, '\n');
+          /* If there is a \n, it must be at the end.  */
+          TEST_VERIFY (pnl == NULL || pnl == line_buffer + ret - 1);
+          fputs (line_buffer, result.out);
+
+          /* Restore the original read size if required.  */
+          if (line_buffer_size > read_size && !read_size_preserve)
+            {
+              line_buffer_size = read_size;
+              free (line_buffer);
+              line_buffer = xmalloc (line_buffer_size);
+            }
+        }
+    }
+
+  xfclose (fp);
+  free (fp_buffer);
+  free (line_buffer);
+
+  xfclose_memstream (&result);
+  TEST_VERIFY (result.length == expected_length);
+  TEST_VERIFY (strcmp (result.buffer, expected_contents) == 0);
+  if (test_verbose > 0)
+    {
+      printf ("info: expected (%zu): [[%s]]\n",
+              expected_length, expected_contents);
+      printf ("info:   actual (%zu): [[%s]]\n", result.length, result.buffer);
+    }
+  free (result.buffer);
+}
+
+/* Test one test file with multiple read parameters.  */
+static void
+test_one_file (void)
+{
+  for (buffer_size = -1; buffer_size <= maximum_line_length + 1; ++buffer_size)
+    for (read_size = 2; read_size <= maximum_line_length + 2; ++read_size)
+      for (read_size_increment = 1; read_size_increment <= 4;
+           ++read_size_increment)
+        for (read_size_preserve = 0; read_size_preserve < 2;
+             ++read_size_preserve)
+          run_test ();
+}
+
+
+static int
+do_test (void)
+{
+  /* Set up the test file contents.  */
+  for (line_lengths[0] = -1; line_lengths[0] <= maximum_line_length;
+       ++line_lengths[0])
+    for (line_lengths[1] = -1; line_lengths[1] <= maximum_line_length;
+         ++line_lengths[1])
+      for (line_lengths[2] = -1; line_lengths[2] <= maximum_line_length;
+           ++line_lengths[2])
+        for (no_newline_at_eof = 0; no_newline_at_eof < 2; ++no_newline_at_eof)
+          {
+            if (!write_test_file ())
+              continue;
+            test_one_file ();
+          }
+  free (test_file_path);
+  return 0;
+}
+
+#define PREPARE prepare
+#include <support/test-driver.c>