expose mmap size and offset option (#141)

* expose mmap size and offset option

* fix panics

2 files changed, 18 insertions, 5 deletions

diff --git a/src/javascript/jsc/api/bun.zig b/src/javascript/jsc/api/bun.zig
index 660f2aba8..de85d6c8f 100644
--- a/src/javascript/jsc/api/bun.zig
+++ b/src/javascript/jsc/api/bun.zig
@@ -1619,18 +1619,29 @@ pub fn mmapFile(
     const file_flags: u32 = if (@hasDecl(std.os.MAP, "FILE")) std.os.MAP.FILE else 0;
 
     // Conforming applications must specify either MAP_PRIVATE or MAP_SHARED.
+    var offset: usize = 0;
     var flags = file_flags;
+    var map_size: ?usize = null;
 
     if (args.nextEat()) |opts| {
         const sync = opts.get(ctx.ptr(), "sync") orelse JSC.JSValue.jsBoolean(false);
         const shared = opts.get(ctx.ptr(), "shared") orelse JSC.JSValue.jsBoolean(true);
         flags |= @as(u32, if (sync.toBoolean()) sync_flags else 0);
         flags |= @as(u32, if (shared.toBoolean()) std.os.MAP.SHARED else std.os.MAP.PRIVATE);
+
+        if (opts.get(ctx.ptr(), "size")) |value| {
+            map_size = @intCast(usize, value.toInt64());
+        }
+
+        if (opts.get(ctx.ptr(), "offset")) |value| {
+            offset = @intCast(usize, value.toInt64());
+            offset = std.mem.alignBackwardAnyAlign(offset, std.mem.page_size);
+        }
     } else {
         flags |= std.os.MAP.SHARED;
     }
 
-    const map = switch (JSC.Node.Syscall.mmapFile(buf_z, flags)) {
+    const map = switch (JSC.Node.Syscall.mmapFile(buf_z, flags, map_size, offset)) {
         .result => |map| map,
 
         .err => |err| {
diff --git a/src/javascript/jsc/node/syscall.zig b/src/javascript/jsc/node/syscall.zig
index ac7d51d8b..3a9e3ddd4 100644
--- a/src/javascript/jsc/node/syscall.zig
+++ b/src/javascript/jsc/node/syscall.zig
@@ -486,21 +486,23 @@ fn mmap(
     return Maybe([]align(mem.page_size) u8){ .result = @ptrCast([*]align(mem.page_size) u8, @alignCast(mem.page_size, rc))[0..length] };
 }
 
-pub fn mmapFile(path: [:0]const u8, flags: u32) Maybe([]align(mem.page_size) u8) {
+pub fn mmapFile(path: [:0]const u8, flags: u32, wanted_size: ?usize, offset: usize) Maybe([]align(mem.page_size) u8) {
     const fd = switch (open(path, os.O.RDWR, 0)) {
         .result => |fd| fd,
         .err => |err| return .{ .err = err },
     };
 
-    const size = switch (fstat(fd)) {
+    var size = std.math.sub(usize, @intCast(usize, switch (fstat(fd)) {
         .result => |result| result.size,
         .err => |err| {
             _ = close(fd);
             return .{ .err = err };
         },
-    };
+    }), offset) catch 0;
+
+    if (wanted_size) |size_| size = @minimum(size, size_);
 
-    const map = switch (mmap(null, @intCast(usize, size), os.PROT.READ | os.PROT.WRITE, flags, fd, 0)) {
+    const map = switch (mmap(null, size, os.PROT.READ | os.PROT.WRITE, flags, fd, offset)) {
         .result => |map| map,
 
         .err => |err| {