Only call fwrite() if select() said the fd is ready.

[Sthing]

The current code calls fwrite() even when select_write() returns 0, ie when the write is expected to fail with EAGAIN (Resource temporarily unavailable). This happens if the TCP transmit buffer is full.
In real life I have seen select() return 0 and fwrite() succeeds anyway - I guess some room became available between the two calls.
But at one point fwrite() will fail with EAGAIN - and then an AMQPIOException occurs.

With the proposed changes fwrite() will not result in EAGAIN. Instead the result is an AMQPTimeoutException if the fd is still blocking after $this->write_timeout.

(See #1131 for a question about how the timeout should be calculated).

I don't know if this change should include any new tests.

I have used rabbitmqctl set_vm_memory_high_watermark 0on the RabbitMQ server to force the EAGAIN error when testing this issue.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (73d6fe1) 74.01% compared to head (e5e52a9) 73.71%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1132      +/-   ##
============================================
- Coverage     74.01%   73.71%   -0.31%     
- Complexity     1035     1037       +2     
============================================
  Files            39       39              
  Lines          3125     3127       +2     
============================================
- Hits           2313     2305       -8     
- Misses          812      822      +10     

Files	Coverage Δ
PhpAmqpLib/Wire/IO/SocketIO.php	80.66% <100.00%> (+0.12%)	⬆️
PhpAmqpLib/Wire/IO/StreamIO.php	71.96% <100.00%> (+0.13%)	⬆️

... and 2 files with indirect coverage changes

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[ramunasd]

Thanks, this indeed was missed when select_write() was introduced.

[lukebakken]

Thank you @Sthing and @ramunasd !