test(profiling): allocation_sampling_distance regression

Author: morrisonlevi

profiling/tests/phpt/allocation_sampling_distance.phpt

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+--TEST--
+[profiling] allocation sampling distance is configurable
+--DESCRIPTION--
+This code path had a regression, so it seems worth adding a test to ensure it
+cannot regress again.
+--SKIPIF--
+<?php
+if (!extension_loaded('datadog-profiling'))
+    die("skip: test requires datadog-profiling");
+?>
+--INI--
+datadog.profiling.enabled=1
+datadog.profiling.allocation_enabled=1
+datadog.profiling.allocation_sampling_distance=1
+datadog.profiling.log_level=trace
+datadog.profiling.output_pprof=/tmp/profiling-data/pprof
+zend.assertions=1
+assert.exception=1
+--FILE--
+<?php
+// Goal: trigger a smallish allocation so we won't run afoul of the default
+// sampling distance, but also something unique-ishly sized so we can be
+// reasonably sure that the log corresponds to our inputs and not accidentally
+// something else.
+//
+// A zend_string costs 24 bytes on 64-bit architectures just for the struct,
+// plus we need the string data and null.
+//     24 + strlen($str) + 1
+// We want a number so that the total number of bytes is evenly divisible by
+// 16 to avoid worrying about rounding/padding that can apply. So let's target
+// 112 total bytes which is divisible by 16 and seems relatively unique-ish:
+//     112 = 24 + strlen($str) + 1
+//     112 - 24 - 1 = strlen($str)
+//               87 = strlen($str)
+// ... except that the engine does the rounding in the wrong place for
+// str_repeat, so it with our inputs it ends up doing this, with `f` being the
+// function which determines the rounded amount:
+//     1 * 87 + f(24 + 0 + 1)
+//         87 + f(25)
+//         87 + 32 = 119
+// So it over-allocates by 7 bytes, and the log will have 119 bytes.
+// Our regex allows for 112 (correct) and 119 (observed).
+$str = \str_repeat('a', 87);
+?>
+Done.
+--EXPECTREGEX--
+.* Memory allocation profiling initialized with a sampling distance of 1 bytes.*
+.* Sent stack sample of [0-9]* frames, .* labels, 11[2,9] bytes allocated, and 1 allocations to profiler.*
+.*Done\..*