Re: Fix some error handling for read() and errno

Hello.

At Sun, 20 May 2018 09:05:22 +0900, Michael Paquier <michael@paquier.xyz> wrote in <20180520000522.GB1603@paquier.xyz>
> Hi all,
> 
> This is basically a new thread after what has been discussed for
> pg_controldata with its error handling for read():
> https://www.postgresql.org/message-id/CABUevEx8ZRV5Ut_FvP2etXiQppx3xVzm7oOaV3AcdHxX81Yt8Q%40mail.gmail.com
> 
> While reviewing the core code, I have noticed similar weird error
> handling for read().  At the same time, some of those places may use an
> incorrect errno, as an error is invoked using an errno which may be
> overwritten by another system call.  I found a funny one in slru.c,
> for which I have added a note in the patch.  I don't think that this is
> worth addressing with more facility, thoughts are welcome.
> 
> Attached is a patch addressing the issues I found.

I see the same issue in snapbuild.c(4 places).

| readBytes = read(fd, &ondisk, SnapBuildOnDiskConstantSize);
| pgstat_report_wait_end();
| if (readBytes != SnapBuildOnDiskConstantSize)
| {
|   CloseTransientFile(fd);
|   ereport(ERROR,
|       (errcode_for_file_access(),
|        errmsg("could not read file \"%s\", read %d of %d: %m",
|           path, readBytes, (int) SnapBuildOnDiskConstantSize)));
| }

and walsender.c (2 places)

|   if (nread <= 0)
|     ereport(ERROR,
|         (errcode_for_file_access(),
|          errmsg("could not read file \"%s\": %m",
|             path)));

and pg_receivewal.c

| if (read(fd, (char *) buf, sizeof(buf)) != sizeof(buf))
| {
|   fprintf(stderr, _("%s: could not read compressed file \"%s\": %s\n"),
|       progname, fullpath, strerror(errno));

pg_waldump.c

| if (readbytes <= 0)
...
|   fatal_error("could not read from log file %s, offset %u, length %d: %s",
|         fname, sendOff, segbytes, strerror(err));


A bit differnt issue, but in pg_waldump.c, search_directory can
check uninitialized errno when read returns a non-zero value.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center