less memory thrashing

src/cmph.zig

@@ -28,6 +28,7 @@ pub fn pack(allocator: Allocator, input: [][*:0]const u8) Error![]const u8 {
 
     const size = c.cmph_packed_size(hash);
     var buf = try allocator.alloc(u8, size);
+    errdefer allocator.free(buf);
     c.cmph_pack(hash, &buf[0]);
     c.cmph_destroy(hash);
     return buf[4..];

src/group.zig

@@ -139,6 +139,7 @@ pub fn someMembers(
     members: []const []const u8,
 ) Allocator.Error!BufSet {
     var bufset = BufSet.init(allocator);
+    errdefer bufset.deinit();
     for (members) |member| {
         try bufset.insert(member);
     }

src/sections.zig

@@ -53,6 +53,7 @@ const Corpus = struct {
     ) initErr!Corpus {
         var arena = std.heap.ArenaAllocator.init(baseAllocator);
         var allocator = arena.allocator();
+        errdefer arena.deinit();
 
         var users = try allocator.alloc(User, usersConst.len);
         var groups = try allocator.alloc(Group, groupsConst.len);
@@ -108,6 +109,7 @@ const Corpus = struct {
         var username2groups = StringHashMap(
             ArrayListUnmanaged(*const Group),
         ).init(baseAllocator);
+        defer username2groups.deinit();
 
         for (groups) |*group| {
             var members = try allocator.alloc(*const User, group.members.count());
@@ -150,7 +152,6 @@ const Corpus = struct {
             const usergroups = elem.value_ptr.*.toOwnedSlice(allocator);
             try username2groups_final.put(username, usergroups);
         }
-        username2groups.deinit();
 
         return Corpus{
             .arena = arena,
@@ -243,11 +244,13 @@ pub const UserGids = struct {
 
 const userGidsErr = Allocator.Error || error{Overflow};
 pub fn userGids(allocator: Allocator, corpus: *const Corpus) userGidsErr!UserGids {
-    var arena = std.heap.ArenaAllocator.init(allocator);
-    defer arena.deinit();
     var blob = ArrayList(u8).init(allocator);
-
+    errdefer blob.deinit();
     var name2offset = StringHashMap(u32).init(allocator);
+    errdefer name2offset.deinit();
+
+    var scratch = try allocator.alloc(u32, 256);
+    defer allocator.free(scratch);
     for (corpus.users) |user| {
         const usergroups_maybe = corpus.username2groups.get(user.name);
         if (usergroups_maybe == null)
@@ -255,19 +258,16 @@ pub fn userGids(allocator: Allocator, corpus: *const Corpus) userGidsErr!UserGid
         const usergroups = usergroups_maybe.?;
 
         try name2offset.putNoClobber(user.name, try math.cast(u32, blob.items.len));
-        var deltaCompressedGids = try arena.allocator().alloc(u32, usergroups.len);
-
-        deltaCompressedGids.len = usergroups.len;
-        for (usergroups) |group, i| {
-            deltaCompressedGids[i] = group.gid;
-        }
-        compress.deltaCompress(u32, deltaCompressedGids) catch |err| switch (err) {
+        scratch = try allocator.realloc(scratch, usergroups.len);
+        scratch.len = usergroups.len;
+        for (usergroups) |group, i|
+            scratch[i] = group.gid;
+        compress.deltaCompress(u32, scratch) catch |err| switch (err) {
             error.NotSorted => unreachable,
         };
         try compress.appendUvarint(&blob, usergroups.len);
-        for (deltaCompressedGids) |gid| {
+        for (scratch) |gid|
             try compress.appendUvarint(&blob, gid);
-        }
     }
 
     return UserGids{