log stats 0, and pardes re-pins cloud9 whose 9ns opens such a file direct and nonseekable: fs.md's follow recipe with cat works through a mount

The log stated the length an open would freeze, a length a mount's
kernel could take as its end, and a splicing cat read it from the
kernel's offset. log now stats 0, as the other streams and generated
views do, and the re-pinned cloud9's 9ns opens any file stating 0
FOPEN_DIRECT_IO | FOPEN_NONSEEKABLE. A look at such a file (the mounted
index) reads a regular file that will not seek as a stream rather than
refusing it as a pipe. selfmount runs the recipe, exec 3<>log; echo
follow >&3; cat <&3, through the mount and hears a record made after it
started.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

3 files changed, 18 insertions, 14 deletions

diff --git a/src/fs.zig b/src/fs.zig
index d13e36bd..91edd429 100644
--- a/src/fs.zig
+++ b/src/fs.zig
@@ -1360,7 +1360,13 @@ fn readFileLimit(gpa: std.mem.Allocator, path: []const u8, limit: usize) ![]u8 {
     defer _ = libc.close(fd);
 
     const end = libc.lseek(fd, 0, libc.SEEK.END);
-    if (end < 0 and libc.errno(end) == .SPIPE) return error.NotAFile;
+    // Not seekable: a pipe or a FIFO is no file; a regular file that is a
+    // stream (a 9ns mount's log or screen, opened nonseekable) is read to
+    // its end, as one with no length is.
+    if (end < 0 and libc.errno(end) == .SPIPE) {
+        const facts = ninep_io.statNoFollow(path_z) orelse return error.NotAFile;
+        if (facts.mode & 0o170000 != 0o100000) return error.NotAFile;
+    }
     const size: usize = if (end < 0) 0 else @intCast(end);
     if (end >= 0 and libc.lseek(fd, 0, libc.SEEK.SET) < 0) return error.ReadFailed;
     if (size == 0) {
diff --git a/src/ninep/events.zig b/src/ninep/events.zig
index bacf3ad6..9752e287 100644
--- a/src/ninep/events.zig
+++ b/src/ninep/events.zig
@@ -494,18 +494,6 @@ pub fn writeLog(p: *Pardes, req: Req) Reply {
     return .{ .tag = req.tag, .written = @intCast(req.data.len) };
 }
 
-/// The length a stat reports: the text an open would freeze now.
-pub fn logLen(q: *const Queue) u64 {
-    var n: u64 = 0;
-    var at = q.head;
-    while (at + 4 <= q.buf.items.len) {
-        const len = std.mem.readInt(u32, q.buf.items[at..][0..4], .little);
-        n += len;
-        at += 4 + len;
-    }
-    return n;
-}
-
 // ---- per-pane event records ----
 
 pub const max_record_text = 256;
diff --git a/src/ninep/tree.zig b/src/ninep/tree.zig
index be03e8f0..8c536c51 100644
--- a/src/ninep/tree.zig
+++ b/src/ninep/tree.zig
@@ -646,7 +646,9 @@ fn topSize(p: *Pardes, f: TopFile) u64 {
         .README => fs.help.len,
         .status => ctl.statusLen(p),
         .look, .exec, .tagexec => ctl.resultsLen(p),
-        .log => events.logLen(&p.fs.log),
+        // A stream: a length would read as its end to a kernel that caches
+        // one (a 9ns mount's splice), and a follower's reads go past it.
+        .log => 0,
         .ctl => ctl.rootLen(p),
         .commands => ctl.commandsLen(),
         .focus => focus: {
@@ -1335,6 +1337,14 @@ test "filesystem inspection preserves pending and displayed Look hover" {
     try testing.expect(!p.raw_hover_intent);
 }
 
+test "log stats 0 however much it holds, a stream as acme's generated files are" {
+    const p = try th.withFile(testing.allocator, "x\n");
+    defer p.deinit();
+    _ = th.wr(p, Node.of(th.serialOf(p), .exec), "Msg a record for the log\n");
+    try testing.expect(th.logHas(p, "a record for the log"));
+    try testing.expectEqual(@as(u64, 0), call(p, .{ .tag = 1, .op = .getattr, .node = @intFromEnum(TopFile.log) }).reply.attr.size);
+}
+
 test "filesystem pane creation and truncation cancel Look hover" {
     const requests = [_]Req{
         // Opening `new` is what makes a pane now; walking to it makes none,