[atharvamhaske]

Fixes #2088.

awk 'BEGIN { print "AWK" }' printed its output and then waited to read stdin until EOF. vm.process.exec leaves stdin open, so the call ran until its timeout and returned timed_out with exit 137.

POSIX awk exits after a program made only of BEGIN actions without reading input. The wrapper in software/gawk/native/crates/awk/src/lib.rs now checks the parsed program. If every rule is a BEGIN rule, it runs the program with no input. awk-rs returns 0 for a plain getline and never feeds it the main input, so skipping the input loses nothing.

A BEGIN-only program also no longer opens the files named on the command line. awk 'BEGIN { print "x" }' /nonexistent now prints x and exits 0 instead of failing with exit 2. macOS awk does the same.

Testing

• cargo test -p agentos-awk: the new begin_only_programs_do_not_read_input test checks that BEGIN-only programs skip input and that programs with an END rule, a main rule or a pattern still read it.
• I built cmd-awk on the host and ran awk 'BEGIN { print "AWK" }' with its stdin pipe held open. The patched binary printed AWK and exited 0 in 0.68 s. The binary built from main had not exited after 10 minutes.
• printf 'a b\nc d\n' | awk '{ print $2 } END { print NR }' still prints b, d, 2.

I did not rerun the issue's VM repro. The issue also notes that stdin: '' does not help. That points to vm.process.exec not closing the guest's stdin when given an empty string, which this PR does not change.