From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>
Cc: pgsql-hackers@postgresql.org, Kunshchikov Vladimir <Vladimir.Kunshchikov@infotecs.ru>
Subject: Re: pg_dump, gzwrite, and errno
Date: Thu, 18 Jun 2020 17:30:18 -0400
Message-ID: <1597085.1592515818@sss.pgh.pa.us> (raw)
In-Reply-To: <20200618212027.GA17103@alvherre.pgsql>
References: <20200618212027.GA17103@alvherre.pgsql>
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
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To: pgsql-hackers@postgresql.org
Cc: tgl@sss.pgh.pa.us, alvherre@2ndquadrant.com, pryzby@telsasoft.com, Vladimir.Kunshchikov@infotecs.ru
Subject: Re: pg_dump, gzwrite, and errno
In-Reply-To: <1597085.1592515818@sss.pgh.pa.us>
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