build: harden low-memory selfdev builds

Author: 1jehuang

docs/COMPILE_PERFORMANCE_PLAN.md

@@ -157,16 +157,25 @@ Use it when capturing comparable before/after numbers for refactors.
   - warm touched-file `selfdev-jcode` build: **18.874s**
 - 2026-04-28: diagnosed the repeated self-dev `jcode` lib build `SIGTERM` on this 16 GiB,
   no-swap workstation. `journalctl -u earlyoom` showed earlyoom sending `SIGTERM` to the root
-  `rustc` at about **1.09 GiB RSS** when available memory crossed the 10% threshold. A direct
-  no-`sccache` build reproduced the same signal, so `sccache` was only reporting the termination.
-  `scripts/dev_cargo.sh` now enables adaptive low-memory overrides for `--profile selfdev` when
-  Linux + earlyoom + no swap + <24 GiB RAM + <8 GiB currently available RAM are detected:
-  `CARGO_INCREMENTAL=0`, `CARGO_PROFILE_SELFDEV_INCREMENTAL=false`, and
-  `CARGO_PROFILE_SELFDEV_CODEGEN_UNITS=16`. Use `JCODE_SELFDEV_LOW_MEMORY=off` to disable, or
-  `JCODE_SELFDEV_LOW_MEMORY=on` to force. Validation: the original root build completed under
-  those settings in **2m34s** after the interrupted partial build reused artifacts; a later
-  benchmark with 9.4 GiB available showed that preserving the inherited selfdev profile can reduce
-  warm edit builds from about **60s** to about **14s** when there is enough headroom.
+  `rustc` when available memory crossed the 10% threshold. A direct no-`sccache` build reproduced
+  the same signal, so `sccache` was only reporting the termination. `scripts/dev_cargo.sh` now
+  enables adaptive low-memory overrides for `--profile selfdev` when Linux + earlyoom + no swap +
+  <24 GiB RAM + <8 GiB currently available RAM are detected: `CARGO_INCREMENTAL=1`,
+  `CARGO_PROFILE_SELFDEV_INCREMENTAL=true`, and `CARGO_PROFILE_SELFDEV_CODEGEN_UNITS=256`. Use
+  `JCODE_SELFDEV_LOW_MEMORY=off` to disable, or `JCODE_SELFDEV_LOW_MEMORY=on` to force. Initial
+  validation completed under the earlier settings in **2m34s** after an interrupted partial build
+  reused artifacts; a later benchmark with 9.4 GiB available showed that preserving the inherited
+  selfdev profile can reduce warm edit builds from about **60s** to about **14s** when there is
+  enough headroom.
+- 2026-05-21: rechecked the same failure mode during overnight TUI test triage. `journalctl -u
+  earlyoom` showed repeated `SIGTERM` events against root `jcode` `rustc` processes at about
+  **2.7-3.3 GiB RSS**. `CARGO_PROFILE_SELFDEV_CODEGEN_UNITS=16` still failed under current browser
+  and desktop-session memory pressure, while a direct no-`sccache` selfdev build with incremental
+  enabled and `CARGO_PROFILE_SELFDEV_CODEGEN_UNITS=256` completed. The adaptive low-memory default
+  was changed to match that passing profile, disables `sccache` by default because `sccache`
+  rejects Cargo incremental builds, and `scripts/dev_cargo.sh` now honors `SCCACHE_DISABLE=1`
+  before auto-enabling the wrapper. Fresh lib-test compilation remains heavier than the binary
+  build and may still require a remote builder, more free memory, or swap.
 - 2026-05-05: trimmed root compile surface by replacing broad `tokio/full` with explicit used
   features, aligning Jcode-owned `crossterm` dependencies on 0.29, and replacing `qr2term` with
   direct `qrcode` rendering. This removed the duplicate `crossterm 0.28` path from the `jcode`

scripts/dev_cargo.sh

@@ -28,6 +28,17 @@ append_rustflags() {
 }
 
 maybe_enable_sccache() {
+  case "${SCCACHE_DISABLE:-}" in
+    1|true|yes|on)
+      if [[ -n "${RUSTC_WRAPPER:-}" && "${RUSTC_WRAPPER}" == *sccache* ]]; then
+        unset RUSTC_WRAPPER
+      fi
+      sccache_status="disabled-by-env"
+      log "sccache disabled by SCCACHE_DISABLE"
+      return
+      ;;
+  esac
+
   if [[ -n "${RUSTC_WRAPPER:-}" ]]; then
     sccache_status="external:${RUSTC_WRAPPER}"
     log "keeping existing RUSTC_WRAPPER=${RUSTC_WRAPPER}"
@@ -174,7 +185,7 @@ selfdev_low_memory_default_needed() {
   [[ -n "$mem_total_kib" && -n "$mem_available_kib" && -n "$swap_total_kib" ]] || return 1
 
   # On small no-swap machines, earlyoom can terminate the root jcode rustc
-  # around 1 GiB RSS before the kernel OOM killer would report anything.
+  # around 1-3 GiB RSS before the kernel OOM killer would report anything.
   # Keep this adaptive so larger workstations, and currently-idle smaller
   # workstations with enough headroom, retain the faster inherited selfdev
   # profile by default.
@@ -207,9 +218,14 @@ maybe_configure_low_memory_selfdev() {
       ;;
   esac
 
-  export CARGO_INCREMENTAL="${CARGO_INCREMENTAL:-0}"
-  export CARGO_PROFILE_SELFDEV_INCREMENTAL="${CARGO_PROFILE_SELFDEV_INCREMENTAL:-false}"
-  export CARGO_PROFILE_SELFDEV_CODEGEN_UNITS="${CARGO_PROFILE_SELFDEV_CODEGEN_UNITS:-16}"
+  export CARGO_INCREMENTAL="${CARGO_INCREMENTAL:-1}"
+  export CARGO_PROFILE_SELFDEV_INCREMENTAL="${CARGO_PROFILE_SELFDEV_INCREMENTAL:-true}"
+  export CARGO_PROFILE_SELFDEV_CODEGEN_UNITS="${CARGO_PROFILE_SELFDEV_CODEGEN_UNITS:-256}"
+  # The low-memory profile deliberately keeps incremental compilation enabled
+  # to avoid repeatedly rebuilding large root crates. sccache rejects Cargo
+  # incremental builds, so default to direct rustc unless the caller opts out
+  # of low-memory mode entirely.
+  export SCCACHE_DISABLE="${SCCACHE_DISABLE:-1}"
   selfdev_low_memory_status="enabled:incremental=${CARGO_PROFILE_SELFDEV_INCREMENTAL},codegen-units=${CARGO_PROFILE_SELFDEV_CODEGEN_UNITS}"
   log "using low-memory selfdev overrides (${selfdev_low_memory_status#enabled:})"
 }