a5d42e58d7f8ce1e985cf7422145619122ca5d25
braney
  Sat Sep 19 18:16:16 2026 -0700
mallocTopPadTester: check the heap step setting still reaches the library

cfgSetMallocTopPad() asks glibc to grow the heap in steps of mallocTopPad bytes
instead of its 128 kB default, which saves a heavy hgTracks render more than a
hundred thousand system calls that draw nothing.

The failure to catch is backsliding: the mallopt call is dropped in a later
edit, or the setting is renamed, or it stops being read before the first
allocation.  Every page still draws correctly and the only symptom is that
renders are slower than they used to be, which nobody attributes to this.

So the test measures the step, not the clock.  A timing test here would be red
on a busy hgwdev, green on an idle one, and deleted within a month.  sbrk(0)
says where the heap ends and the distance it jumps when malloc extends it is
what M_TOP_PAD sets, so the answer is the same on a loaded machine as an idle
one.  It is read in buckets, since glibc may round and may change what it
rounds to; what must not change is the order of magnitude between a configured
step and the default.  Allocations stay under the mmap threshold, or the break
never moves and the test measures nothing while appearing to pass.

Two runs, with the setting and without, because the answer is the difference
between them: a single run would pass on a library whose own default happened
to be large.

Watched to fail and then pass: leaving the knob read but never applied gives
the default step under both confs, which is the one line the diff shows.
Recorded as sandbox-ab in utils/testRegistry.

refs #38225, refs #38391

diff --git src/hg/lib/tests/expected/mallocTopPadTest src/hg/lib/tests/expected/mallocTopPadTest
new file mode 100644
index 00000000000..2b489874b5c
--- /dev/null
+++ src/hg/lib/tests/expected/mallocTopPadTest
@@ -0,0 +1,4 @@
+mallocTopPad in hg.conf: (not set)
+first heap growth:       under 4 MB, the library's own default step
+mallocTopPad in hg.conf: 33554432
+first heap growth:       16 MB or more, a configured step