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Improve the granularity of PQsocketPoll's timeout parameter.

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Commit f5e4dedfa exposed libpq's internal function PQsocketPoll
without a lot of thought about whether that was an API we really
wanted to chisel in stone.  The main problem with it is the use of
time_t to specify the timeout.  While we do want an absolute time
so that a loop around PQsocketPoll doesn't have problems with
timeout slippage, time_t has only 1-second resolution.  That's
already problematic for libpq's own internal usage --- for example,
pqConnectDBComplete has long had a kluge to treat "connect_timeout=1"
as 2 seconds so that it doesn't accidentally round to nearly zero.
And it's even less likely to be satisfactory for external callers.
Hence, let's change this while we still can.

The best idea seems to be to use an int64 count of microseconds since
the epoch --- basically the same thing as the backend's TimestampTz,
but let's use the standard Unix epoch (1970-01-01) since that's more
likely for clients to be easy to calculate.  Millisecond resolution
would be plenty for foreseeable uses, but maybe the day will come that
we're glad we used microseconds.

Also, since time(2) isn't especially helpful for computing timeouts
defined this way, introduce a new function PQgetCurrentTimeUSec
to get the current time in this form.

Remove the hack in pqConnectDBComplete, so that "connect_timeout=1"
now means what you'd expect.

We can also remove the "#include <time.h>" that f5e4dedfa added to
libpq-fe.h, since there's no longer a need for time_t in that header.
It seems better for v17 not to enlarge libpq-fe.h's include footprint
from what it's historically been, anyway.

I also failed to resist the temptation to do some wordsmithing
on PQsocketPoll's documentation.

Patch by me, per complaint from Dominique Devienne.

Discussion: https://postgr.es/m/913559.1718055575@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2024-06-13 15:14:32 -04:00
parent 6dfac24401
commit 105024a472
8 changed files with 153 additions and 78 deletions
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121
doc/src/sgml/libpq.sgml
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@ -262,41 +262,6 @@ PGconn *PQsetdb(char *pghost,
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsocketPoll">
<term><function>PQsocketPoll</function><indexterm><primary>PQsocketPoll</primary></indexterm></term>
<listitem>
<para>
<indexterm><primary>nonblocking connection</primary></indexterm>
Poll a connection&apos;s underlying socket descriptor retrieved with <xref linkend="libpq-PQsocket"/>.
<synopsis>
int PQsocketPoll(int sock, int forRead, int forWrite, time_t end_time);
</synopsis>
</para>
<para>
This function sets up polling of a file descriptor. The underlying function is either
<function>poll(2)</function> or <function>select(2)</function>, depending on platform
support. The primary use of this function is iterating through the connection sequence
described in the documentation of <xref linkend="libpq-PQconnectStartParams"/>. If
<parameter>forRead</parameter> is specified, the function waits for the socket to be ready
for reading. If <parameter>forWrite</parameter> is specified, the function waits for the
socket to be ready for write. See <literal>POLLIN</literal> and <literal>POLLOUT</literal>
from <function>poll(2)</function>, or <parameter>readfds</parameter> and
<parameter>writefds</parameter> from <function>select(2)</function> for more information. If
<parameter>end_time</parameter> is not <literal>-1</literal>, it specifies the time at which
this function should stop waiting for the condition to be met.
</para>
<para>
The function returns a value greater than <literal>0</literal> if the specified condition
is met, <literal>0</literal> if a timeout occurred, or <literal>-1</literal> if an error
occurred. The error can be retrieved by checking the <literal>errno(3)</literal> value. In
the event <literal>forRead</literal> and <literal>forWrite</literal> are not set, the
function immediately returns a timeout condition.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQconnectStartParams">
<term><function>PQconnectStartParams</function><indexterm><primary>PQconnectStartParams</primary></indexterm></term>
<term><function>PQconnectStart</function><indexterm><primary>PQconnectStart</primary></indexterm></term>
@ -546,6 +511,70 @@ switch(PQstatus(conn))
</listitem>
</varlistentry>
<varlistentry id="libpq-PQsocketPoll">
<term><function>PQsocketPoll</function><indexterm><primary>PQsocketPoll</primary></indexterm></term>
<listitem>
<para>
<indexterm><primary>nonblocking connection</primary></indexterm>
Poll a connection's underlying socket descriptor retrieved with
<xref linkend="libpq-PQsocket"/>.
The primary use of this function is iterating through the connection
sequence described in the documentation of
<xref linkend="libpq-PQconnectStartParams"/>.
<synopsis>
typedef pg_int64 pg_usec_time_t;
int PQsocketPoll(int sock, int forRead, int forWrite,
pg_usec_time_t end_time);
</synopsis>
</para>
<para>
This function performs polling of a file descriptor, optionally with
a timeout.
If <parameter>forRead</parameter> is nonzero, the
function will terminate when the socket is ready for
reading. If <parameter>forWrite</parameter> is nonzero,
the function will terminate when the
socket is ready for writing.
</para>
<para>
The timeout is specified by <parameter>end_time</parameter>, which
is the time to stop waiting expressed as a number of microseconds since
the Unix epoch (that is, <type>time_t</type> times 1 million).
Timeout is infinite if <parameter>end_time</parameter>
is <literal>-1</literal>. Timeout is immediate (no blocking) if
end_time is <literal>0</literal> (or indeed, any time before now).
Timeout values can be calculated conveniently by adding the desired
number of microseconds to the result of
<xref linkend="libpq-PQgetCurrentTimeUSec"/>.
Note that the underlying system calls may have less than microsecond
precision, so that the actual delay may be imprecise.
</para>
<para>
The function returns a value greater than <literal>0</literal> if the
specified condition is met, <literal>0</literal> if a timeout occurred,
or <literal>-1</literal> if an error occurred. The error can be
retrieved by checking the <literal>errno(3)</literal> value. In the
event both <parameter>forRead</parameter>
and <parameter>forWrite</parameter> are zero, the function immediately
returns a timeout indication.
</para>
<para>
<function>PQsocketPoll</function> is implemented using either
<function>poll(2)</function> or <function>select(2)</function>,
depending on platform. See <literal>POLLIN</literal>
and <literal>POLLOUT</literal> from <function>poll(2)</function>,
or <parameter>readfds</parameter> and
<parameter>writefds</parameter> from <function>select(2)</function>,
for more information.
</para>
</listitem>
</varlistentry>
<varlistentry id="libpq-PQconndefaults">
<term><function>PQconndefaults</function><indexterm><primary>PQconndefaults</primary></indexterm></term>
<listitem>
@ -1390,8 +1419,7 @@ postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
<para>
Maximum time to wait while connecting, in seconds (write as a decimal integer,
e.g., <literal>10</literal>). Zero, negative, or not specified means
wait indefinitely. The minimum allowed timeout is 2 seconds, therefore
a value of <literal>1</literal> is interpreted as <literal>2</literal>.
wait indefinitely.
This timeout applies separately to each host name or IP address.
For example, if you specify two hosts and <literal>connect_timeout</literal>
is 5, each host will time out if no connection is made within 5
@ -8039,6 +8067,25 @@ int PQlibVersion(void);
</listitem>
</varlistentry>
<varlistentry id="libpq-PQgetCurrentTimeUSec">
<term><function>PQgetCurrentTimeUSec</function><indexterm><primary>PQgetCurrentTimeUSec</primary></indexterm></term>
<listitem>
<para>
Retrieves the current time, expressed as the number of microseconds
since the Unix epoch (that is, <type>time_t</type> times 1 million).
<synopsis>
pg_usec_time_t PQgetCurrentTimeUSec(void);
</synopsis>
</para>
<para>
This is primarily useful for calculating timeout values to use with
<xref linkend="libpq-PQsocketPoll"/>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>