test: fix two flaky tests (darwin start_time regex, memstats HeapReleased drift) (#2050)

* test(process_collector): match scientific-notation start time on darwin

TestDarwinProcessCollector asserted process_start_time_seconds against [0-9.]{10,}, but expfmt renders large gauge values in scientific notation (for example 1.783414e+09), whose leading digit run is shorter than ten characters. The check then fails intermittently on macOS depending on the process start time. Match a leading digit like the sibling metric checks, which asserts the metric is present with a numeric value.

Signed-off-by: Kemal Akkoyun <kemal.akkoyun@datadoghq.com>

* test(collector): stabilize TestMemStatsEquivalence against HeapReleased drift

The test read runtime.ReadMemStats and runtime/metrics separately and compared them within 5%, with the code itself noting it hoped no GC ran in between. HeapReleased is driven by the background scavenger and drifted between the two reads, failing intermittently (seen on Linux arm64). Force a full scavenge with debug.FreeOSMemory and disable GC for the measurement window so both reads observe the same runtime state.

Signed-off-by: Kemal Akkoyun <kemal.akkoyun@datadoghq.com>

---------

Signed-off-by: Kemal Akkoyun <kemal.akkoyun@datadoghq.com>

Author: kakkoyun

prometheus/go_collector_latest_test.go

@@ -21,6 +21,7 @@ import (
 	"reflect"
 	"regexp"
 	"runtime"
+	"runtime/debug"
 	"runtime/metrics"
 	"strings"
 	"sync"
@@ -270,12 +271,16 @@ func TestMemStatsEquivalence(t *testing.T) {
 		samplesMap[descs[i].Name] = &samples[i]
 	}
 
-	// Force a GC cycle to try to reach a clean slate.
-	runtime.GC()
+	// Reach a stable slate and hold it for a single measurement window.
+	// FreeOSMemory runs a GC and returns freed memory to the OS, leaving the
+	// background scavenger nothing to release; disabling GC for the window
+	// keeps msReal and msFake observing the same runtime state. Without this,
+	// HeapReleased could drift between the two reads and fail the comparison.
+	debug.FreeOSMemory()
+	defer debug.SetGCPercent(debug.SetGCPercent(-1))
 
-	// Populate msReal.
+	// Populate msReal, then msFake back-to-back within the frozen window.
 	runtime.ReadMemStats(&msReal)
-	// Populate msFake and hope that no GC happened in between (:
 	metrics.Read(samples)
 
 	memStatsFromRM(&msFake, samplesMap)

prometheus/process_collector_darwin_test.go

@@ -55,12 +55,12 @@ func TestDarwinProcessCollector(t *testing.T) {
 		regexp.MustCompile("\nprocess_max_fds [1-9]"),
 		regexp.MustCompile("\nprocess_open_fds [1-9]"),
 		regexp.MustCompile("\nprocess_virtual_memory_max_bytes (-1|[1-9])"),
-		regexp.MustCompile("\nprocess_start_time_seconds [0-9.]{10,}"),
+		regexp.MustCompile("\nprocess_start_time_seconds [0-9]"),
 		regexp.MustCompile("\nfoobar_process_cpu_seconds_total [0-9]"),
 		regexp.MustCompile("\nfoobar_process_max_fds [1-9]"),
 		regexp.MustCompile("\nfoobar_process_open_fds [1-9]"),
 		regexp.MustCompile("\nfoobar_process_virtual_memory_max_bytes (-1|[1-9])"),
-		regexp.MustCompile("\nfoobar_process_start_time_seconds [0-9.]{10,}"),
+		regexp.MustCompile("\nfoobar_process_start_time_seconds [0-9]"),
 	} {
 		if !re.Match(buf.Bytes()) {
 			t.Errorf("want body to match %s\n%s", re, buf.String())