Add tee-compatible append, ignore-interrupts, and output-error options

Adopt the parts of tee(1)'s option vocabulary that map onto t3's
command-runner model, so t3 is more familiar as a tee replacement (issue #1).

- -a, --append: open the log file with "a" so output is appended rather than
  overwriting, e.g. to accumulate a log across multiple build steps.

- -i, --ignore-interrupts: t3 and its timestamp workers ignore SIGINT (set
  before the workers are forked, so they inherit it) while the command child
  restores the default disposition. A Ctrl-C therefore interrupts the command
  but lets t3 drain the workers' remaining output and exit with the command's
  status, instead of being torn down mid-flush and dropping the buffered tail.

- --output-error[=MODE]: choose what t3 does when a write to one of its
  outputs fails - warn, warn-nopipe, exit, exit-nopipe, and tee's default
  (exit on a broken pipe, diagnose other errors). SIGPIPE is ignored so a
  closed consumer yields EPIPE for this logic rather than killing t3, and a
  failed output is dropped while the others keep going - so
  --output-error=warn-nopipe preserves the complete log file even when a
  downstream reader of t3's own stdout/stderr (e.g. `t3 log -- cmd | head`)
  goes away. The logfile's deferred write errors are reported at close.

t3's existing -p (--plain) deliberately does not take on tee's pipe-mode
meaning; pipe behavior is reached through --output-error=...-nopipe instead.
This divergence is noted in the README and man page.

Add golden tests for -a and --output-error, and document all three options in
the usage text, README, and man page.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: limeytexan

Makefile

@@ -34,7 +34,7 @@ $(MAN1DIR)/%: %
 	cp $< $@
 	chmod 444 $@
 
-.PHONY: all install lint clean stress bench
+.PHONY: all install lint format clean stress options-test test bench
 all: $(BIN) $(MAN1)
 
 install: $(INSTBIN) $(INSTMAN1)
@@ -159,8 +159,13 @@ stress: $(BIN) tests/stress tests/run-stress tests/check-stream.awk
 	@T3=./$(BIN) STRESS_THREADS=$(STRESS_THREADS) STRESS_LINES=$(STRESS_LINES) \
 	    tests/run-stress
 
-# Run the stress test as part of the standard `make test` suite.
-test: stress
+# Behavioral tests for the tee-compatible options (--append, --output-error)
+# that the golden-file harness cannot express.
+options-test: $(BIN) tests/run-options
+	@T3=./$(BIN) tests/run-options
+
+# Run the stress and option tests as part of the standard `make test` suite.
+test: stress options-test
 
 # Benchmark: estimate t3's marginal per-line overhead. Manual only - not part
 # of `make test`. Tunable, e.g. `make bench COUNT=2000000 WIDTHS="32 128"`.

README.md

@@ -125,11 +125,36 @@ Invoke provided command and write its colorized, precise time-stamped output bot
   -e, --errcolor C  set the ANSI escape sequence used to color stderr
   -t, --ts          enable timestamps in all outputs
   -r, --relative    display timestamps as relative offsets from start time (implies --ts)
+  -a, --append      append to the log file instead of overwriting it
+  -i, --ignore-interrupts  ignore interrupt signals (finish flushing output on Ctrl-C)
+  --output-error[=MODE]    set behavior on a write error; MODE is one of
+                    warn, warn-nopipe, exit, exit-nopipe (a bare --output-error
+                    means warn; with no --output-error, t3 exits on a broken
+                    pipe and warns on other write errors)
   -h, --help        print this help message
   -v, --version     print version string
   --debug           enable debugging
 ```
 
+The `-a`/`--append`, `-i`/`--ignore-interrupts`, and `--output-error` options
+take their names and broad meaning from [`tee(1)`](https://www.gnu.org/software/coreutils/tee),
+with a few deliberate differences noted below.
+
+- **`-i`/`--ignore-interrupts`** does more than `tee`'s: it makes `t3` *and* its
+  timestamp worker processes ignore `SIGINT`, while the wrapped command keeps
+  the default disposition (so a `Ctrl-C` still interrupts the command). `t3`
+  then drains and flushes the command's final output before exiting, rather
+  than being torn down mid-flush.
+- **`--output-error`** — by default, matching `tee`, `t3` **exits** when a write
+  to its own stdout or stderr fails with a broken pipe, so the log file can be
+  left truncated if a downstream consumer (e.g. `t3 log -- cmd | head`) closes
+  early. On such a fatal write error `t3` exits with a failure status, which
+  **replaces the wrapped command's own exit status**. Pass
+  **`--output-error=warn-nopipe`** to instead ignore broken-pipe errors and keep
+  the log file (and the surviving stream) complete.
+- **`-p`** remains `t3`'s `--plain`, *not* `tee`'s pipe-mode flag; reach the
+  pipe-aware behavior through `--output-error=…-nopipe`.
+
 ## Installing
 
 The easiest way to get `t3` is using Flox: