emit build-database for IDEs: one failing member loses the whole database, and build programs cannot tell a plan pass #699

mcpp-language-server gets a project's model from mcpp emit build-database --format json. Reproducing Sunrisepeak/mcpp-language-server#23 on a real 5-member Windows workspace (GalTranslPP: vcpkg + Qt 6, mcpp 2026.9.25.1, windows-2025) showed two properties of emit that cost the editor its model. Both are behaviour the spec states today, so this is a request, not a bug report. Tracked on our side as UP-M2 / UP-M3 in Sunrisepeak/mcpp-language-server#24.

1. One member's failure loses every set (SPEC-005 R5.2)

• GPPGUI's build program calls mcpp::dep_bin("gpp.updater", "Updater"). Building that host tool ran its own lupdate check action, which failed on that machine (lupdate.exe could not load Qt6Qml.dll).
• Result: MCPP_BUILD_DATABASE_PLAN_FAILED: GPPGUI: building host tool 'updater:Updater' failed, and no data — the four members that planned fine got no database either.
• Effect: the language server falls back to a guessed model for the whole workspace (a toolchain it found, no include directories); clangd then crashed repeatedly on it.

Ask: when some members plan and others fail, return data with the sets that planned, plus an error diagnostic per failed member, under a code a consumer can tell apart from a total failure (e.g. MCPP_BUILD_DATABASE_PARTIAL).

2. A build program cannot tell it is running for a plan (SPEC-005 R2.5)

• emit runs build.mcpp exactly as build does and builds dependencies' host tools, with no signal in the MCPP_* contract.
• A build program that checks the full environment (a Qt root, a vcpkg .lib per library) — or a host tool whose build runs project checks, as above — fails emit on every machine that cannot build yet. That is exactly when an editor most needs the model.
• mcpp::action edges already do not run under emit (R2.2); the problem is work done in the program itself and in host-tool builds.

Ask (any of):

• a documented signal, e.g. MCPP_PLAN_ONLY=1 / mcpp::plan_only(), so a program can skip checks and heavy work while planning;
• under emit, a build-program failure becomes a diagnostic for that member instead of failing the command (pairs with 1);
• under emit, do not run the check actions a host-tool build declares.

Evidence

• Sunrisepeak/GalTranslPP PR feat: compile .c sources via a separate C rule #1 (temporary CI): run 36164741493 fails as above (Qt without qtdeclarative); run 36178100611 plans all 227 units once Qt is complete.
• Related: feat: mcpp emit build-database —— 不写工程目录的 S1 构建数据库（模块图、标准库模块、监视输入） #636, emit build-database 供 IDE 调用：离线缺依赖的错误码、规划期间不泄漏调用方的管道、xlings 调用要有期限（附联网效应、auto_refresh、索引镜像） #648.