[200_22] 修复read-line无法检测到Ctrl+D的问题

[da-liii]

No description provided.

[greptile-apps]

Greptile Summary

This PR fixes a one-line bug in src/s7.c where stdin_read_line was returning nil_string (the empty string "") instead of eof_object when fgets returned NULL (i.e., when Ctrl+D / EOF was pressed on stdin). This made (eof-object? (read-line)) always return #f when reading from stdin, breaking the R7RS contract for read-line.

Key changes:

• In stdin_read_line, the fallback return value is changed from nil_string to eof_object, making it consistent with file_read_line which already returns eof_object in the same situation.
• No new tests are added specifically for stdin EOF detection (existing tests in tests/scheme/base/read-line-test.scm only cover string ports).

Confidence Score: 5/5

This PR is safe to merge — it is a minimal, targeted one-line fix that aligns stdin_read_line with the existing behavior of file_read_line and the R7RS standard.

The change is a single-line fix with clear motivation: fgets returning NULL means EOF (or error), and the correct R7RS return value is the EOF object, not an empty string. The same pattern already exists in file_read_line (line 26958), so this is simply making stdin_read_line consistent. No regressions are expected.

No files require special attention. The change is isolated to a single return statement in stdin_read_line.

Important Files Changed

Filename	Overview
src/s7.c	One-line fix: stdin_read_line now returns eof_object instead of nil_string when fgets returns NULL, consistent with file_read_line and R7RS semantics.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["read-line called"] --> B{"Port type?"}
    B -->|stdin| C["stdin_read_line()"]
    B -->|file| D["file_read_line()"]
    B -->|string| E["string_read_line()"]

    C --> F["fgets(buf, size, stdin)"]
    F -->|"returns non-NULL"| G["return s7_make_string(buf)"]
    F -->|"returns NULL (Ctrl+D / EOF)"| H["return eof_object ✅\n(previously: nil_string ❌)"]

    D --> I["fgets(buf, size, file)"]
    I -->|"returns non-NULL"| J["parse until newline,\nreturn string"]
    I -->|"returns NULL (EOF/error)"| K["return eof_object"]

    style H fill:#d4edda,stroke:#28a745

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