milestone 13 discrepancies: redact credentials from urls in logs, metrics and cli output

src/main.zig

@@ -18,13 +18,35 @@ comptime {
     _ = @import("cli.zig");
 }
 
+/// Streaming, never positional, and this is load-bearing rather than a default
+/// worth changing back.
+///
+/// `File.writer` builds a `.positional` writer: it keeps an offset of its own,
+/// starting at zero, and pwrites there. `std.log` writes to the same descriptor
+/// directly and shares the descriptor's offset. With stderr redirected to a
+/// regular file the two disagree about where the end is, so the runner's first
+/// flush lands on top of whatever the log already wrote at the start and the
+/// operator loses those lines. `writerStreaming` uses the descriptor's own
+/// offset, so both writers append.
+///
+/// A terminal and a pipe are unaffected either way, because neither is seekable
+/// and the positional writer falls back to streaming on them. `2>file` is where
+/// it shows, and `nxdns run 2>logfile` is an ordinary way to run this program.
+/// `>>` needs it too: pwrite ignores `O_APPEND`.
+///
+/// Takes the `File` rather than naming stdout and stderr inside, so a test can
+/// hand it a real file and read back what an operator would have.
+fn runnerWriter(file: std.Io.File, io: std.Io, buffer: []u8) std.Io.File.Writer {
+    return file.writerStreaming(io, buffer);
+}
+
 pub fn main(init: std.process.Init) u8 {
     // Both `File.Writer` values are self-referential and must not move, so they
     // stay in these `var` slots for the whole of `main`.
     var out_buffer: [4096]u8 = undefined;
     var err_buffer: [4096]u8 = undefined;
-    var out = std.Io.File.stdout().writer(init.io, &out_buffer);
-    var err = std.Io.File.stderr().writer(init.io, &err_buffer);
+    var out = runnerWriter(std.Io.File.stdout(), init.io, &out_buffer);
+    var err = runnerWriter(std.Io.File.stderr(), init.io, &err_buffer);
 
     const runner: cli.Runner = .{
         .io = init.io,
@@ -55,3 +77,39 @@ pub fn main(init: std.process.Init) u8 {
         .help => cli.runHelp(runner),
     };
 }
+
+// ---------------------------------------------------------------------------
+// tests
+// ---------------------------------------------------------------------------
+
+test "a runner writer appends after what std.log already put in a redirected file" {
+    // Reproduced live before this test existed: `nxdns run --config <bad file>
+    // 2> file` lost `info(migrations): config.db migrated from schema version 0
+    // to 2`, because the runner's error writer pwrote its first flush at offset
+    // zero, over the line the log had already written. Down a pipe the same run
+    // kept both lines, which is why a terminal never showed this.
+    var threaded: std.Io.Threaded = .init(std.testing.allocator, .{});
+    defer threaded.deinit();
+    const io = threaded.io();
+    const gpa = std.testing.allocator;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+    var file = try tmp.dir.createFile(io, "stderr.txt", .{});
+    defer file.close(io);
+
+    // What the log sink does: a direct write on the shared descriptor, which
+    // moves the descriptor's offset. A positional writer does not see it.
+    const log_line = "info(migrations): config.db migrated from schema version 0 to 2\n";
+    try file.writeStreamingAll(io, log_line);
+
+    const runner_line = "nxdns run failed: MissingDefaultGroup\n";
+    var buffer: [4096]u8 = undefined;
+    var w = runnerWriter(file, io, &buffer);
+    try w.interface.writeAll(runner_line);
+    try w.interface.flush();
+
+    const seen = try tmp.dir.readFileAlloc(io, "stderr.txt", gpa, .limited(4096));
+    defer gpa.free(seen);
+    try std.testing.expectEqualStrings(log_line ++ runner_line, seen);
+}