From 1dfdd3cdbdce1edbbe0388f7fdb50907e31ecd44 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Motiejus=20Jak=C5=A1tys?= <motiejus@jakstys.lt>
Date: Thu, 4 Aug 2022 22:51:55 -0700
Subject: [PATCH] iterator: make advance/rollback more understandable

untested
---
 src/PackedGroup.zig | 22 +++++++++-------------
 src/PackedUser.zig  | 22 +++++++++-------------
 src/libnss.zig      |  8 ++++----
 3 files changed, 22 insertions(+), 30 deletions(-)

diff --git a/src/PackedGroup.zig b/src/PackedGroup.zig
index a59f4ee..2f5dfc8 100644
--- a/src/PackedGroup.zig
+++ b/src/PackedGroup.zig
@@ -69,26 +69,22 @@ pub const Iterator = struct {
     next_start: usize = 0,
     idx: u32 = 0,
     total: u32,
-    advance_by: usize = 0,
+    advanced_by: usize = 0,
 
     pub fn next(it: *Iterator) ?PackedGroup {
-        const gr = it.peek() orelse return null;
-        it.advance();
-        return gr;
-    }
-
-    pub fn peek(it: *Iterator) ?PackedGroup {
         if (it.idx == it.total) return null;
         const entry = fromBytes(it.section[it.next_start..]);
-        it.advance_by = entry.end;
+        it.idx += 1;
+        it.next_start += entry.end;
+        it.advanced_by = entry.end;
         return entry.group;
     }
 
-    pub fn advance(it: *Iterator) void {
-        assert(it.advance_by > 0);
-        it.idx += 1;
-        it.next_start += it.advance_by;
-        it.advance_by = 0;
+    pub fn rollback(it: *Iterator) void {
+        assert(it.advanced_by > 0);
+        it.idx -= 1;
+        it.next_start -= it.advanced_by;
+        it.advanced_by = 0;
     }
 };
 
diff --git a/src/PackedUser.zig b/src/PackedUser.zig
index 492c151..99af6df 100644
--- a/src/PackedUser.zig
+++ b/src/PackedUser.zig
@@ -109,26 +109,22 @@ pub const Iterator = struct {
     shell_reader: ShellReader,
     idx: u32 = 0,
     total: u32,
-    advance_by: usize = 0,
+    advanced_by: usize = 0,
 
     pub fn next(it: *Iterator) ?PackedUser {
-        const u = it.peek() orelse return null;
-        it.advance();
-        return u;
-    }
-
-    pub fn peek(it: *Iterator) ?PackedUser {
         if (it.idx == it.total) return null;
         const entry = fromBytes(it.section[it.next_start..]);
-        it.advance_by = entry.end;
+        it.idx += 1;
+        it.next_start += entry.end;
+        it.advanced_by = entry.end;
         return entry.user;
     }
 
-    pub fn advance(it: *Iterator) void {
-        assert(it.advance_by > 0);
-        it.idx += 1;
-        it.next_start += it.advance_by;
-        it.advance_by = 0;
+    pub fn rollback(it: *Iterator) void {
+        assert(it.advanced_by > 0);
+        it.idx -= 1;
+        it.next_start -= it.advanced_by;
+        it.advanced_by = 0;
     }
 };
 
diff --git a/src/libnss.zig b/src/libnss.zig
index 14bbbfb..b20b260 100644
--- a/src/libnss.zig
+++ b/src/libnss.zig
@@ -370,7 +370,7 @@ fn getgrent_r(
     defer state.getgrent_iterator_mu.unlock();
 
     if (state.getgrent_iterator) |*it| {
-        const group = it.peek() orelse {
+        const group = it.next() orelse {
             errnop.* = 0;
             return c.NSS_STATUS_NOTFOUND;
         };
@@ -381,11 +381,11 @@ fn getgrent_r(
             state.file.db.packCGroup(&group, buffer[0..buflen]);
 
         if (cgroup1) |cgroup| {
-            it.advance();
             result.* = cgroup;
             return c.NSS_STATUS_SUCCESS;
         } else |err| switch (err) {
             error.BufferTooSmall => {
+                it.rollback();
                 errnop.* = @enumToInt(os.E.RANGE);
                 return c.NSS_STATUS_TRYAGAIN;
             },
@@ -423,7 +423,7 @@ fn getpwent_r(
     defer state.getpwent_iterator_mu.unlock();
 
     if (state.getpwent_iterator) |*it| {
-        const user = it.peek() orelse {
+        const user = it.next() orelse {
             errnop.* = 0;
             return c.NSS_STATUS_NOTFOUND;
         };
@@ -431,12 +431,12 @@ fn getpwent_r(
         const buf = buffer[0..buflen];
         const cuser = state.file.db.writeUser(user, buf) catch |err| switch (err) {
             error.BufferTooSmall => {
+                it.rollback();
                 errnop.* = @enumToInt(os.E.RANGE);
                 return c.NSS_STATUS_TRYAGAIN;
             },
         };
 
-        it.advance();
         result.* = cuser;
         return c.NSS_STATUS_SUCCESS;
     } else {