Skip unnecessary source database refresh for unchanged files

When redo-ifchange encounters a source file from within a .do file, it
previously called initializeSourceDatabase unconditionally — even if the
file hadn't changed. That function deletes and recreates the entire
database directory, which opens a corruption window: if the process is
killed (e.g. Ctrl+C triggering SIGKILL via the process group handler)
between the delete and the markSource write, the database is left without
a source marker. This causes permanent "No rule to build" errors in
projects with a catch-all default.do (like Adamant).

Now we compare the current file stamp against the cached stamp first. If
they match, we skip the refresh entirely — the database is already in the
correct state. This eliminates the corruption window for the vast majority
of source files on incremental builds (only files that actually changed
need the refresh).

Includes tests verifying:
- Unchanged sources skip DB refresh (inode stability check)
- Changed sources still trigger refresh and dependent rebuilds
- New sources get properly initialized

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: Gus

src/Build.hs

@@ -120,8 +120,24 @@ redoIfChange = buildTargets redoIfChange'
       runFromDo <- isRunFromDoFile
       case (source, runFromDo) of
         (True, False) -> targetSourceWarning target
-        (True, True) -> do initializeSourceDatabase key target
-                           return ExitSuccess
+        -- Source file encountered from within a .do file. We need to ensure
+        -- the database is marked as source with a current stamp. However,
+        -- if the stamp hasn't changed since last time, we can skip the
+        -- expensive initializeSourceDatabase call (which deletes and recreates
+        -- the entire database directory). This is important because that
+        -- delete-recreate cycle opens a corruption window: if the process is
+        -- killed (e.g. Ctrl+C -> SIGKILL) between the delete and the
+        -- markSource write, the database is left without a source marker,
+        -- causing permanent "No rule to build" errors for the file.
+        -- By skipping unchanged sources, we eliminate this window for the
+        -- vast majority of source files on incremental builds.
+        (True, True) -> do
+          currentStamp <- safeStampTarget target
+          cachedStamp <- getStamp key
+          if currentStamp == cachedStamp
+            then return ExitSuccess
+            else do initializeSourceDatabase key target
+                    return ExitSuccess
         (False, _) -> do
           currentStamp <- safeStampTarget target
           modified <- isTargetModified key currentStamp

test/375-default-do-source/all.do

@@ -0,0 +1,2 @@
+redo clean
+sh run_tests.sh

test/375-default-do-source/clean.do

@@ -0,0 +1 @@
+rm -rf build src *.log

test/375-default-do-source/default.do

@@ -0,0 +1,25 @@
+# Catch-all default.do that mimics a typical project structure:
+# NOTE: no shebang — redo adds "sh -e" automatically, so errors propagate.
+# - Handles targets in build/ directory by depending on corresponding source
+# - Handles "clean" and "all" redo targets
+# - Errors on anything else (source files should never reach here)
+
+case "$1" in
+    build/*)
+        # Build targets: depend on the corresponding source file
+        BASENAME=$(basename "$1")
+        redo-ifchange "src/$BASENAME"
+        echo "built from: $(cat "src/$BASENAME")" > "$3"
+        ;;
+    clean)
+        rm -rf build src *.log
+        ;;
+    all)
+        # Run the test script
+        sh run_tests.sh
+        ;;
+    *)
+        echo "default.do: No rule to build '$1'." >&2
+        exit 1
+        ;;
+esac

test/375-default-do-source/run_tests.sh

@@ -0,0 +1,74 @@
+#!/bin/sh
+# Test for source database refresh optimization.
+#
+# This test directory has a catch-all default.do that handles build/*
+# targets and errors on everything else. The test verifies that
+# initializeSourceDatabase is skipped for unchanged source files
+# (the DB directory is not deleted and recreated unnecessarily),
+# while still refreshing when source files actually change.
+
+set -e
+
+##############################################################################
+# Helpers
+##############################################################################
+get_db_dir() {
+    DB_KEY=$(printf '%s' "$1" | md5sum | awk '{print toupper($1)}')
+    echo "$HOME/.redo/database/$(echo $DB_KEY | cut -c1-3)/$(echo $DB_KEY | cut -c4-9)/$(echo $DB_KEY | cut -c10-21)/$(echo $DB_KEY | cut -c22-)"
+}
+get_stamp_dir() {
+    DB_KEY=$(printf '%s' "$1" | md5sum | awk '{print toupper($1)}')
+    echo "$HOME/.redo/stamps/$(echo $DB_KEY | cut -c1-3)/$(echo $DB_KEY | cut -c4-9)/$(echo $DB_KEY | cut -c10-21)/$(echo $DB_KEY | cut -c22-)"
+}
+
+##############################################################################
+# Setup
+##############################################################################
+mkdir -p src build
+SRC_PATH="$(cd src && pwd)/data.txt"
+BUILD_PATH="$(cd build && pwd)/data.txt"
+rm -rf "$(get_db_dir "$SRC_PATH")" "$(get_stamp_dir "$SRC_PATH")"
+rm -rf "$(get_db_dir "$BUILD_PATH")" "$(get_stamp_dir "$BUILD_PATH")"
+../flush-cache
+
+echo "content v1" > src/data.txt
+
+##############################################################################
+# Initial build — creates source DB with stamp
+##############################################################################
+redo-ifchange build/data.txt
+test "$(cat build/data.txt)" = "built from: content v1" || exit 1
+
+SRC_DB=$(get_db_dir "$SRC_PATH")
+test -d "$SRC_DB/y" || exit 2
+
+# Record the DB directory inode to detect if it gets recreated
+INODE_BEFORE=$(stat -c %i "$SRC_DB" 2>/dev/null || stat -f %i "$SRC_DB" 2>/dev/null)
+
+##############################################################################
+# Rebuild with no changes — DB should NOT be recreated
+##############################################################################
+../flush-cache
+redo-ifchange build/data.txt
+
+test -d "$SRC_DB/y" || exit 3
+
+INODE_AFTER=$(stat -c %i "$SRC_DB" 2>/dev/null || stat -f %i "$SRC_DB" 2>/dev/null)
+if [ "$INODE_BEFORE" != "$INODE_AFTER" ]; then
+    echo "FAIL: source DB was recreated on unchanged rebuild (inode $INODE_BEFORE -> $INODE_AFTER)" >&2
+    exit 4
+fi
+
+##############################################################################
+# Modify source — DB should refresh and target should rebuild
+##############################################################################
+../sleep
+echo "content v2" > src/data.txt
+../flush-cache
+
+redo-ifchange build/data.txt
+test "$(cat build/data.txt)" = "built from: content v2" || exit 5
+test -d "$SRC_DB/y" || exit 6
+
+echo "PASS: source DB skip optimization" >&2
+rm -rf src build