fix(read): assign the variables at end of input, and never a readonly one

[LESdylan]

What & why

This is the loop that also reads a last line with no newline:

while IFS= read -r line || [ -n "$line" ]; do ...; done

On hellish it never ended. At end of input, with nothing read, read returned 1 without assigning anything. $line still held the last line, so the || kept the loop going. bash assigns the variables empty (and empties an -a array) whenever read returns, EOF included. The hellishrc_plugins envload/envdiff had to work around this with an explicit line= before each read.

Root cause. read_one_line returns NULL for "EOF, nothing read", and builtin_read returned on NULL before rd_dispatch. It now dispatches an empty line, so every variable is assigned. The status is still 1.

A second bug this made reachable. read never checked readonly: readonly r=keep; echo x | read r changed r. rd_set_var now refuses a readonly name with bash's r: readonly variable. The statuses are bash's:

• a readonly variable before the last one: 2, and the variables after it are not assigned;
• the last one: 1;
• REPLY: 2;
• an -a array: 1.

read is not a special builtin, so the shell goes on.

How I verified it

• Reproduced first. tests/scripts/58_read_eof_readonly.sh is graded against bash --posix 5.3.9. It covers:

  • EOF with nothing read, for named variables, REPLY, -a, -d, -n and -N;
  • a last line without a newline, and the || [ -n "$line" ] loop (guarded, so a regression fails instead of hanging);
  • readonly in each position, REPLY, -a, and the messages.

  develop diverges on 16 of its 26 lines, and without the guard it loops forever. This branch matches bash byte for byte.

• Local gates on this commit (ASan debug build):

  • golden tests/tester: 5363/5363;
  • tests/run_scripts.sh against bash --posix: 125/125;
  • verify_alloc.sh: identical output on both heaps;
  • alloc_stress.sh: all clean;
  • tests/pty_suite.sh: 110 ok, 6 skipped, 3 failed. The three are prompt_compat_matrix, prompt_drift_matrix and prompt_jobs_badge. They expect the non-root %/$ prompt and get #, because the container runs as root; they fail the same way on develop, and CI runs them as a normal user.

• norminette is OK on every touched file.

Notes / trade-offs

• Found while writing the hellishrc_plugins test suites, not from an issue.

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https://claude.ai/code/session_01RAmeHfJNm7XjYbMNrQqvkG

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