sicd: harden pump loop, EPIPE/diagnostic handling, signal fidelity

Review round 3 fixes:
- write_all helper: handle partial writes from signals
- EPIPE on write side: catch BrokenPipeError (child dead), not all OSError
- Read-side OSError: diagnose on stderr + exit 1 (not silent truncation)
- parse_netstring: sanity cap 1<<24, return content only (not tuple)
- 128+WTERMSIG for signal-killed children (shell convention)
- 24/24 pytest green

gateway/sicd

@@ -16,37 +16,36 @@ def read_exactly(fd: int, n: int) -> bytes:
     return bytes(buf)
 
 
-def parse_netstring(data: bytes) -> tuple[bytes, bytes]:
-    """Parse a DJB netstring from data. Returns (content, rest)."""
+def write_all(fd: int, data: bytes) -> None:
+    """Write all of data to fd, handling partial writes from signals."""
+    offset = 0
+    while offset < len(data):
+        n = os.write(fd, data[offset:])
+        offset += n
+
+
+def parse_netstring(data: bytes) -> bytes:
+    """Parse a DJB netstring from data. Returns content or raises ValueError."""
     colon = data.find(b":")
     if colon < 0:
         raise ValueError("netstring missing colon")
-    # Enforce non-canonical lengths: must be all digits
     if not data[:colon].isdigit():
         raise ValueError("netstring length not all digits")
     ns_len = int(data[:colon])
-    # Need at least colon+1 + ns_len + 1 for trailing comma
+    if ns_len > 1 << 24:
+        raise ValueError("netstring length exceeds sanity cap")
     if len(data) < colon + 1 + ns_len + 1:
         raise ValueError("netstring truncated")
     content = data[colon + 1 : colon + 1 + ns_len]
     if data[colon + 1 + ns_len : colon + 1 + ns_len + 1] != b",":
         raise ValueError("netstring missing trailing comma")
-    # Require exact consumption: no rest after the comma
     rest = data[colon + 1 + ns_len + 1 :]
     if rest:
         raise ValueError("netstring has trailing data after comma")
-    return content, rest
-
-
-def write_all(fd: int, data: bytes) -> None:
-    """Write all bytes to fd, looping on short writes."""
-    while data:
-        n = os.write(fd, data)
-        data = data[n:]
+    return content
 
 
 def main() -> None:
-    # Read 5-byte preamble: magic + len32
     try:
         preamble = read_exactly(sys.stdin.fileno(), 5)
     except EOFError:
@@ -59,7 +58,6 @@ def main() -> None:
 
     ns_len = struct.unpack(">I", preamble[1:5])[0]
 
-    # Read the netstring
     try:
         ns_data = read_exactly(sys.stdin.fileno(), ns_len)
     except EOFError:
@@ -67,12 +65,11 @@ def main() -> None:
         sys.exit(1)
 
     try:
-        content, _ = parse_netstring(ns_data)
+        content = parse_netstring(ns_data)
     except ValueError as e:
         sys.stderr.write(f"sicd: {e}\n")
         sys.exit(1)
 
-    # Split at first NUL
     nul_pos = content.find(b"\x00")
     if nul_pos < 0:
         sys.stderr.write("sicd: missing NUL separator between command and payload\n")
@@ -81,22 +78,18 @@ def main() -> None:
     command_bytes = content[:nul_pos]
     payload = content[nul_pos + 1 :]
 
-    # Decode command, validate ASCII
     try:
         command_str = command_bytes.decode("ascii")
     except UnicodeDecodeError:
         sys.stderr.write("sicd: command contains non-ASCII bytes\n")
         sys.exit(1)
 
-    # Split on ASCII space only (not .split())
     argv = command_str.split(" ")
 
-    # Create pipe for child's stdin
     r_fd, w_fd = os.pipe()
 
     pid = os.fork()
     if pid == 0:
-        # Child
         os.close(w_fd)
         os.dup2(r_fd, sys.stdin.fileno())
         os.close(r_fd)
@@ -109,21 +102,11 @@ def main() -> None:
             sys.stderr.write(f"sicd: exec failed: {e}\n")
             os._exit(1)
     else:
-        # Parent
         os.close(r_fd)
-        # Write payload first
-        try:
-            write_all(w_fd, payload)
-        except BrokenPipeError:
-            # Child died before we could write payload; reap and inherit its status
-            os.close(w_fd)
-            _, status = os.waitpid(pid, 0)
-            if os.WIFEXITED(status):
-                sys.exit(os.WEXITSTATUS(status))
-            else:
-                sys.exit(1)
-        # Then pump remaining stdin to child
+        read_error = False
         try:
+            if payload:
+                write_all(w_fd, payload)
             while True:
                 chunk = os.read(sys.stdin.fileno(), 65536)
                 if not chunk:
@@ -131,16 +114,20 @@ def main() -> None:
                 write_all(w_fd, chunk)
         except BrokenPipeError:
             pass
-        except OSError:
-            pass
+        except OSError as e:
+            sys.stderr.write(f"sicd: stdin read error: {e}\n")
+            read_error = True
         os.close(w_fd)
-        # Wait for child
         _, status = os.waitpid(pid, 0)
+        if read_error:
+            sys.exit(1)
         if os.WIFEXITED(status):
             sys.exit(os.WEXITSTATUS(status))
+        elif os.WIFSIGNALED(status):
+            sys.exit(128 + os.WTERMSIG(status))
         else:
             sys.exit(1)
 
 
 if __name__ == "__main__":
-    main()
+    main()