Extract generate::safe_overwrite helper to eliminate DRY violation

The "safe overwrite" pattern (diff -q tmp dest, then rm or mv) was
duplicated in template, gemfeed (2x), and notes modules. Extracted
into a shared helper with unit tests covering all three cases (no
dest, identical dest, differing dest).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

4 files changed, 41 insertions, 22 deletions

diff --git a/lib/gemfeed.source.sh b/lib/gemfeed.source.sh
index c405083..40792cc 100644
--- a/lib/gemfeed.source.sh
+++ b/lib/gemfeed.source.sh
@@ -20,11 +20,7 @@ gemfeed::updatemainindex () {
         "$gemfeed_dir/index.gmi" >> "$index_gmi.tmp"
 
     # Only overwrite if content changed, preserving mtime for template skip logic
-    if [[ -f "$index_gmi" ]] && diff -q "$index_gmi.tmp" "$index_gmi" >/dev/null 2>&1; then
-        rm "$index_gmi.tmp"
-    else
-        mv "$index_gmi.tmp" "$index_gmi"
-    fi
+    generate::safe_overwrite "$index_gmi.tmp" "$index_gmi"
 }
 
 gemfeed::_get_word_count () {
@@ -65,11 +61,7 @@ GEMFEED
     done < <(gemfeed::get_posts)
 
     # Only overwrite if content changed, preserving mtime for template skip logic
-    if [[ -f "$gemfeed_dir/index.gmi" ]] && diff -q "$gemfeed_dir/index.gmi.tmp" "$gemfeed_dir/index.gmi" >/dev/null 2>&1; then
-        rm "$gemfeed_dir/index.gmi.tmp"
-    else
-        mv "$gemfeed_dir/index.gmi.tmp" "$gemfeed_dir/index.gmi"
-    fi
+    generate::safe_overwrite "$gemfeed_dir/index.gmi.tmp" "$gemfeed_dir/index.gmi"
 
     gemfeed::updatemainindex
 }
diff --git a/lib/generate.source.sh b/lib/generate.source.sh
index fec4f20..544ae08 100644
--- a/lib/generate.source.sh
+++ b/lib/generate.source.sh
@@ -40,6 +40,19 @@ generate::internal_link_id () {
     tr '[:upper:]' '[:lower:]' <<< "$text" | tr "' " '-' | tr -cd 'A-Za-z0-9-'
 }
 
+# Atomically replace a file only if content actually changed, preserving mtime
+# for downstream skip logic. Removes the temp file if unchanged.
+generate::safe_overwrite () {
+    local -r tmp_file="$1"; shift
+    local -r dest_file="$1"; shift
+
+    if [[ -f "$dest_file" ]] && diff -q "$tmp_file" "$dest_file" >/dev/null 2>&1; then
+        rm "$tmp_file"
+    else
+        mv "$tmp_file" "$dest_file"
+    fi
+}
+
 # Add other docs (e.g. images, videos) from Gemtext to output format.
 # Skips copying if the output file already exists and is newer than the source.
 generate::fromgmi_add_docs () {
@@ -301,4 +314,28 @@ generate::draft () {
 generate::test () {
     local text="I can't believe it!"
     assert::equals "$(generate::internal_link_id "$text")" 'i-can-t-believe-it'
+
+    # Test generate::safe_overwrite: dest does not exist, tmp should be moved
+    local tmp_dir=$(mktemp -d)
+    echo 'new content' > "$tmp_dir/file.tmp"
+    generate::safe_overwrite "$tmp_dir/file.tmp" "$tmp_dir/file"
+    assert::equals "$(cat "$tmp_dir/file")" 'new content'
+    assert::equals "$(test -f "$tmp_dir/file.tmp" && echo exists || echo gone)" 'gone'
+
+    # Test generate::safe_overwrite: dest exists and is identical, mtime preserved
+    local before_mtime=$(stat -c '%Y' "$tmp_dir/file")
+    sleep 1
+    echo 'new content' > "$tmp_dir/file.tmp"
+    generate::safe_overwrite "$tmp_dir/file.tmp" "$tmp_dir/file"
+    local after_mtime=$(stat -c '%Y' "$tmp_dir/file")
+    assert::equals "$before_mtime" "$after_mtime"
+    assert::equals "$(test -f "$tmp_dir/file.tmp" && echo exists || echo gone)" 'gone'
+
+    # Test generate::safe_overwrite: dest exists but differs, content replaced
+    echo 'different content' > "$tmp_dir/file.tmp"
+    generate::safe_overwrite "$tmp_dir/file.tmp" "$tmp_dir/file"
+    assert::equals "$(cat "$tmp_dir/file")" 'different content'
+    assert::equals "$(test -f "$tmp_dir/file.tmp" && echo exists || echo gone)" 'gone'
+
+    rm -rf "$tmp_dir"
 }
diff --git a/lib/notes.source.sh b/lib/notes.source.sh
index 85bd2b5..08af3e5 100644
--- a/lib/notes.source.sh
+++ b/lib/notes.source.sh
@@ -42,9 +42,5 @@ That were all notes. Hope they were useful!
 NOTES
 
     # Only overwrite if content changed, preserving mtime for template skip logic
-    if [[ -f "$notes_dir/index.gmi" ]] && diff -q "$notes_dir/index.gmi.tmp" "$notes_dir/index.gmi" >/dev/null 2>&1; then
-        rm "$notes_dir/index.gmi.tmp"
-    else
-        mv "$notes_dir/index.gmi.tmp" "$notes_dir/index.gmi"
-    fi
+    generate::safe_overwrite "$notes_dir/index.gmi.tmp" "$notes_dir/index.gmi"
 }
diff --git a/lib/template.source.sh b/lib/template.source.sh
index 9488ea8..5d9f4dd 100644
--- a/lib/template.source.sh
+++ b/lib/template.source.sh
@@ -71,13 +71,7 @@ template::_generate_file () {
     template::_generate < "$tpl" > "$dest.tmp"
 
     # Only overwrite if content actually changed, preserving mtime for caches
-    if [[ -f "$dest" ]] && diff -q "$dest.tmp" "$dest" >/dev/null 2>&1; then
-        rm "$dest.tmp"
-        log VERBOSE "Template output unchanged for $dest"
-    else
-        mv "$dest.tmp" "$dest"
-        log INFO "Done generating $dest"
-    fi
+    generate::safe_overwrite "$dest.tmp" "$dest"
     cd - >/dev/null
 }