ESP8266: fix heap metric functions and add OOM diagnostics

[joluxer]

Description:

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.8
• The code change is tested and works with Tasmota core ESP32 V.3.3.8 from Platform 2026.05.50
• I accept the CLA.

Summary

Two commits addressing ESP8266 heap observability. Commit 1 is self-contained
and useful independently; Commit 2 builds on the corrected metric functions.

---

Commit 1 - Adapt ESP_getMaxAllocHeap() and ESP_getHeapFragmentation()

Problem:
ESP_getMaxAllocHeap() returned ESP.getFreeHeap() (total free heap, not the
largest contiguous allocatable block).
ESP_getHeapFragmentation() read ummHeapInfo.maxFreeContiguousBlocks, which
is only valid immediately after a umm_info() heap walk - stale or zero otherwise.

Fix:
Both functions now align with their ESP32 equivalents, returning accurate live
values derived from umm_max_block_size() and umm_free_heap_size_lw().

A new ESP_UpdateHeapMetrics() caches the result of umm_max_block_size()
(heap walk, brief IRQ disable). The cache is refreshed:

• once per second in PerformEverySecond() when SetOption130 is active
• on demand in CmndStatus() before Status 4 output

umm_max_block_size() and umm_free_heap_size_lw() are unconditionally
available (UMM_INFO and UMM_STATS are hardcoded in umm_malloc_cfg.h);
no additional build flags are required.

Observable changes:

• SetOption130 heap timestamp on ESP8266 now includes the fragmentation
  percentage. Format changes from HH:MM:SS.mmm-FreeKB to
  HH:MM:SS.mmm-FreeKB/Frag%. Log parsers using the ESP8266-specific
  single-field format will need updating. The new format matches ESP32 behavior.
• Status 4 (StatusMEM) gains two ESP8266-specific fields:
  MaxFreeBlock (KB) and Frag (%).

---

Commit 2 - OOM diagnostics, Status 4 extensions, Status 44 heap dump

Opt-in heap diagnostics gated on UMM_INLINE_METRICS or UMM_STATS_FULL
build flags. Without these flags: no runtime behavior change.

OOM event monitoring (requires UMM_INLINE_METRICS or UMM_STATS_FULL):

• ESP_HeapOomCheck(): called once per second; logs OOM counter delta on
  change (OOM: count N (+M))
• ESP_HeapOomTest(): logs current OOM count on demand

Additional Status 4 fields:

• OomCount - cumulative OOM events (UMM_INLINE_METRICS or UMM_STATS_FULL)
• HeapLwm (KB) - heap low-watermark since boot (UMM_STATS_FULL)
• MaxAllocSz (bytes) - peak single allocation size (UMM_STATS_FULL)

Status 44 (ESP8266-only):

• Triggers umm_info(nullptr, true): prints full heap block map to serial
• Returns {"Status44":{"HeapDump":"serial"}}
• Command number chosen above the standard Status range (0–MAX_STATUS = 0–13)
  and below the reserved value 99 (full status dump)

Enabling the diagnostic flags:
The flags must reach the ESP8266 Arduino framework's umm_malloc allocator,
which is compiled separately from the Tasmota sketch. user_config_override.h
affects only sketch compilation units and cannot reach the framework.
The required mechanism is build_flags in platformio_override.ini:

[env]
build_flags = ${common.build_flags}
              -DUMM_STATS_FULL
              -DUMM_INLINE_METRICS

Example lines are added to platformio_override_sample.ini.

---

Memory impact (ESP8266, tasmota release build)

Scope	Flash	RAM	IRAM
Commit 1 (always active)	+~1960 B	+~32 B	0
Commit 2, no flags	+~84 B	0	0
Commit 2, with UMM_* flags	+~492 B	+~40 B	+~300 B
Total, no flags	+~2044 B	+~32 B	0
Total, with flags	+~2452 B	+~72 B	+~300 B

The +300 B IRAM with flags is attributable to OOM monitoring functions
compiled IRAM-resident. This applies only to explicit diagnostic builds;
standard release builds are unaffected (0 IRAM impact).

---

Tested

Developed and initially tested on Tasmota v15.4.0, ESP-12F.
Build-tested and target-tested on the development branch.
SetOption130 active in both captures below.

Before (upstream development, no flags):

18:36:07.615-024 CMD: Status 4
18:36:07.627-021 MQT: feedback/heizstab/STATUS4 = {"StatusMEM":{"ProgramSize":659,
  "Free":1388,"Heap":24,"ProgramFlashSize":4096,"FlashSize":4096,
  "FlashChipId":"1625C2","FlashFrequency":40,"FlashMode":"DOUT",
  "Features":[...],"Drivers":"...","Sensors":"..."}}

After (this PR, UMM_STATS_FULL + UMM_INLINE_METRICS active):

18:40:08.501-023/02 CMD: Status 4
18:40:08.518-020/00 MQT: feedback/heizstab/STATUS4 = {"StatusMEM":{"ProgramSize":662,
  "Free":1384,"Heap":23,"ProgramFlashSize":4096,"FlashSize":4096,
  "FlashChipId":"1625C2","FlashFrequency":40,"FlashMode":"DOUT",
  "MaxFreeBlock":23,"Frag":1,"OomCount":0,"HeapLwm":18,"MaxAllocSz":4096,
  "Features":[...],"Drivers":"...","Sensors":"..."}}

Visible changes:

• Timestamp format: -024 → -023/02 (FreeKB → FreeKB/Frag%)
• New Status 4 fields: MaxFreeBlock, Frag, OomCount, HeapLwm, MaxAllocSz
• ProgramSize delta: 659 → 662 KB (+3 KB, consistent with measured flash impact)

Status 44 (heap block map printed to serial, excerpt;
heap map format reference:
source - umm_info.c in ESP8266 Arduino core /
docs - rhempel/umm_malloc):

18:42:20.928-023/03 CMD: Status 44

+----------+-------+--------+--------+-------+--------+--------+
|0x3fff24c0|B     0|NB     1|PB     0|Z     1|NF    67|PF  1313|
|0x3fff24c8|B     1|NB     7|PB     0|Z     6|
|0x3fff25e8|B    37|NB    39|PB    28|Z     2|NF   177|PF   220|
|0x3fff3590|B   538|NB  1051|PB   535|Z   513|
|0x3fff63b8|B  2015|NB  4967|PB  1954|Z  2952|NF  1202|PF  1842|
|0x3fffbff8|B  4967|NB     0|PB  2015|Z     1|NF     0|PF     0|
... (128 entries total)
+----------+-------+--------+--------+-------+--------+--------+
Total Entries   128    Used Entries   116    Free Entries    12
Total Blocks   4966    Used Blocks   1936    Free Blocks   3030
+--------------------------------------------------------------+
Usage Metric:                  63
Fragmentation Metric:           3
+--------------------------------------------------------------+
umm heap statistics:
  Raw Free Space    24240
  OOM Count             0
  Low Watermark     17160
  Low Watermark ISR 21400
  MAX Alloc Request  4096
  Size of umm_block     8
+--------------------------------------------------------------+
18:42:20.557-023/03 OOM: count 0 (test-trigger)
18:42:21.002-020/00 MQT: feedback/heizstab/STATUS44 = {"Status44":{"HeapDump":"serial"}}