bug #19510 [Process] Fix double-fread() when reading unix pipes (nicolas-grekas)

This PR was merged into the 2.7 branch.

Discussion
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[Process] Fix double-fread() when reading unix pipes

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

While looking at the blackfire profile of a `composer install`, I was able to reduce the number of calls to `fread` from 90k to 60k using this patch (and from 60k to <1k with https://github.com/composer/composer/pull/5569 but that's another story).

In fact, we should continue reading only if there might be something next, which won"t be the case if the buffer has not been filled.

Commits
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ac17617 [Process] Fix double-fread() when reading unix pipes

src/Symfony/Component/Process/Pipes/UnixPipes.php

@@ -120,7 +120,7 @@ class UnixPipes extends AbstractPipes
             do {
                 $data = fread($pipe, self::CHUNK_SIZE);
                 $read[$type] .= $data;
-            } while (isset($data[0]));
+            } while (isset($data[0]) && ($close || isset($data[self::CHUNK_SIZE - 1])));
 
             if (!isset($read[$type][0])) {
                 unset($read[$type]);