Re: pg_dump, gzwrite, and errno

Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> On 2020-Jun-11, Justin Pryzby wrote:
>> --- a/src/bin/pg_dump/pg_backup_directory.c
>> +++ b/src/bin/pg_dump/pg_backup_directory.c
>> @@ -347,8 +347,12 @@ _WriteData(ArchiveHandle *AH, const void *data, size_t dLen)
>>         lclContext *ctx = (lclContext *) AH->formatData;
>>
>>         if (dLen > 0 && cfwrite(data, dLen, ctx->dataFH) != dLen)
>> +       {
>> +               if (errno == 0)
>> +                       errno = ENOSPC;
>>                 fatal("could not write to output file: %s",
>>                           get_cfp_error(ctx->dataFH));
>> +       }
>>  }

> This seems correct to me.

Surely it's insufficient as-is, because there is no reason to suppose
that errno is zero at entry.  You'd need to set errno = 0 first.

Also it's fairly customary in our sources to include a comment about
this machination; so the full ritual is usually more like

    errno = 0;
    if (pg_pwrite(fd, data, len, xlrec->offset) != len)
    {
        /* if write didn't set errno, assume problem is no disk space */
        if (errno == 0)
            errno = ENOSPC;
        ereport ...

> (I spent a long time looking at zlib sources
> to convince myself that it does work with compressed files too)

Yeah, it's not obvious that gzwrite has the same behavior w.r.t. errno
as a plain write.  But there's not much we can do to improve matters
if it does not, so we might as well assume it does.

            regards, tom lane