fix(changes): Discard takes only the ticked files and blocks (#87)

The Changes panel's "Discard all" threw away every changed file, ticked or
not. It is now "Discard (n)": the same selection Commit sends — whole ticked
files as `paths`, a partly ticked file's ticked blocks as `hunks` — and is
disabled with nothing ticked. The server's `discard` accepts `hunks`
(`selectionPatch` in reverse, `git apply -R`), building every patch before
applying any so a stale selection undoes nothing.

Closes #87

server/lib/git-ops.js

@@ -424,6 +424,13 @@ async function runGitAction(dir, action, body = {}) {
 
   if (action === "discard") {
     if (body.confirm !== true) throw bad("ECONFIRM", "discard requires confirm: true");
+    // Ticked change blocks of partly ticked files (#87) go first: each patch
+    // is built before any is applied, so a stale one refuses before the
+    // irreversible whole-file part below has run.
+    const partial = hasHunks(body) ? await discardSelection(dir, body) : null;
+    if (partial && !(Array.isArray(body.paths) && body.paths.length)) {
+      return done(action, partial);
+    }
     const paths = await resolvePaths(dir, body);
     const untracked = paths.filter((p) => p.code === "??").map((p) => p.path);
     // A rename's ORIGINAL path rides along: restoring only the new one would
@@ -431,7 +438,7 @@ async function runGitAction(dir, action, body = {}) {
     const tracked = paths
       .filter((p) => p.code !== "??")
       .flatMap((p) => (p.from && p.code.includes("R") ? [p.path, p.from] : [p.path]));
-    const out = [];
+    const out = partial ? [partial] : [];
     // Untracked files are deleted (git has no copy to restore); tracked ones
     // go back to HEAD in both gutters, so a staged-then-edited file lands in
     // one state rather than half-reverted.
@@ -825,18 +832,11 @@ function hasHunks(body) {
 const MAX_PARTIAL_FILES = 300;
 
 /**
- * JetBrains' partial commit: whole files (`paths`) plus, per file in `hunks`,
- * only the ticked change blocks — `{"<path>": [{hunk: "@@ … @@", blocks: [0]}]}`,
- * the hunks named as `stage-hunk` names them, at `context`. Built in a
- * TEMPORARY index (HEAD + the whole files + one patch per partial file from
- * `selectionPatch`, git's own fresh text, never the request's) and committed
- * from it, so the real index and the working tree are never in the way; after
- * the commit the real index entries of exactly those paths are reset to the
- * new HEAD, the rest of each partial file left as an unstaged change. Same
- * path rules as `commitPaths`; a partial file must be tracked (EUNTRACKED)
- * and not a rename, and a hunk the tree no longer has is EHUNKGONE.
+ * `body.hunks` checked for shape, and its files resolved as changed paths
+ * that may be taken in part: tracked (EUNTRACKED), not a rename, not in
+ * conflict, and not also named whole in `body.paths`.
  */
-async function commitSelection(dir, args, body) {
+async function partialPicks(dir, body, verb) {
   const picks = Object.entries(body.hunks);
   if (picks.length > MAX_PARTIAL_FILES) throw bad("EBADPATH", "too many files");
   for (const [, list] of picks) {
@@ -858,12 +858,54 @@ async function commitSelection(dir, args, body) {
   const conflicted = [...whole, ...part].find((p) => CONFLICT_CODES.has(p.code));
   if (conflicted) throw bad("ECONFLICTS", `${conflicted.path} is in conflict: resolve it first`);
   for (const p of part) {
-    if (p.code === "??") throw bad("EUNTRACKED", `${p.path} is untracked: commit it whole`);
-    if (p.code.includes("R")) throw bad("EBADPATH", `${p.path} is a rename: commit it whole`);
+    if (p.code === "??") throw bad("EUNTRACKED", `${p.path} is untracked: ${verb} it whole`);
+    if (p.code.includes("R")) throw bad("EBADPATH", `${p.path} is a rename: ${verb} it whole`);
     if (whole.some((w) => w.path === p.path)) {
       throw bad("EBADPATH", `${p.path} is named both whole and in part`);
     }
   }
+  return { picks, whole, part };
+}
+
+/**
+ * The partial discard (#87): per file in `hunks`, only the ticked change
+ * blocks are undone off the working tree — `selectionPatch` in reverse, git's
+ * own fresh text, never the request's — exactly what one `discard-hunk` per
+ * block would do, in one step. Every patch is built before the first is
+ * applied, so a stale selection refuses (EHUNKGONE) with nothing undone.
+ */
+async function discardSelection(dir, body) {
+  const { picks } = await partialPicks(dir, body, "discard");
+  const patches = [];
+  for (const [p, list] of picks) {
+    const patch = await selectionPatch(dir, p, body.context, list, true);
+    if (!patch)
+      throw bad("EHUNKGONE", `${p}: that change is no longer in the diff; refresh and retry`);
+    patches.push(patch);
+  }
+  // The patch names paths from the repo root, so it is applied there.
+  const top = (await git(dir, ["rev-parse", "--show-toplevel"])).stdout.trim();
+  const zero = Number(body.context) === 0 ? ["--unidiff-zero"] : [];
+  const out = [];
+  for (const patch of patches)
+    out.push(await git(top, ["apply", "-R", ...zero, "-"], { input: patch }));
+  return mergeResults(out);
+}
+
+/**
+ * JetBrains' partial commit: whole files (`paths`) plus, per file in `hunks`,
+ * only the ticked change blocks — `{"<path>": [{hunk: "@@ … @@", blocks: [0]}]}`,
+ * the hunks named as `stage-hunk` names them, at `context`. Built in a
+ * TEMPORARY index (HEAD + the whole files + one patch per partial file from
+ * `selectionPatch`, git's own fresh text, never the request's) and committed
+ * from it, so the real index and the working tree are never in the way; after
+ * the commit the real index entries of exactly those paths are reset to the
+ * new HEAD, the rest of each partial file left as an unstaged change. Same
+ * path rules as `commitPaths`; a partial file must be tracked (EUNTRACKED)
+ * and not a rename, and a hunk the tree no longer has is EHUNKGONE.
+ */
+async function commitSelection(dir, args, body) {
+  const { picks, whole, part } = await partialPicks(dir, body, "commit");
   const wholePaths = whole.flatMap((p) =>
     p.from && p.code.includes("R") ? [p.path, p.from] : [p.path]
   );