Fix IO/FS/read_line when the line ends with a EOF

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project does not currently adhere to a particular versioning scheme.
 - Fix readback when hvm net has `a{n}` or `x{n}` vars. ([#659][gh-659])
 - Fix imported constructors not being updated to Constructor expression. ([#674][gh-674])
 - Fix parse error on parenthesized eraser. ([#675][gh-675])
+- Fix IO/FS/read_line when the line ends with a EOF. ([#638][gh-638])
 
 ### Added
 
@@ -426,4 +427,5 @@ and this project does not currently adhere to a particular versioning scheme.
 [gh-673]: https://github.com/HigherOrderCO/Bend/pull/673
 [gh-674]: https://github.com/HigherOrderCO/Bend/issues/674
 [gh-675]: https://github.com/HigherOrderCO/Bend/issues/675
+[gh-638]: https://github.com/HigherOrderCO/Bend/issues/638
 [Unreleased]: https://github.com/HigherOrderCO/Bend/compare/0.2.36...HEAD

src/fun/builtins.bend

@@ -307,17 +307,26 @@ def IO/FS/read_line.read_chunks(fd, chunks):
     # Read line in 1kB chunks
     chunk <- IO/done_on_err(IO/FS/read(fd, 1024))
     match res = List/split_once(chunk, '\n'):
+      # Found a newline, backtrack and join chunks
       case Result/Ok:
         (line, rest) = res.val
         (length, *) = List/length(rest)
         * <- IO/FS/seek(fd, to_i24(length) * -1, IO/FS/SEEK_CUR)
         chunks = List/Cons(line, chunks)
         bytes = List/flatten(chunks)
         return wrap(bytes)
+      # Newline not found
       case Result/Err:
         line = res.val
-        chunks = List/Cons(line, chunks)
-        return IO/FS/read_line.read_chunks(fd, chunks)
+        (length, line) = List/length(line)
+        # If length is 0, the end of the file was reached, return as if it was a newline
+        if length == 0:
+          bytes = List/flatten(chunks)
+          return wrap(bytes)
+        # Otherwise, the line is still ongoing, read more chunks
+        else:
+          chunks = List/Cons(line, chunks)
+          return IO/FS/read_line.read_chunks(fd, chunks)
 
 # IO/FS/write_file(path: String, bytes: (List u24)) -> (IO None)
 # Writes a list of bytes to a file given by a path.

tests/golden_tests/io/eof.txt

@@ -0,0 +1 @@
+text

tests/golden_tests/io/read_line_eof.bend

@@ -0,0 +1,6 @@
+def main:
+  with IO:
+    fd <- IO/done_on_err(IO/FS/open("tests/golden_tests/io/eof.txt", "r"))
+    bytes <- IO/FS/read_line(fd)
+    txt = String/decode_utf8(bytes)
+    return wrap(txt)

tests/snapshots/io__read_line_eof.bend.snap

@@ -0,0 +1,6 @@
+---
+source: tests/golden_tests.rs
+input_file: tests/golden_tests/io/read_line_eof.bend
+---
+Strict mode:
+λa (a IO/Done/tag IO/MAGIC "text")