[stdlib] Fix input() segfaults on EOF

stdlib/src/builtin/io.mojo

@@ -67,7 +67,7 @@ struct _fdopen[mode: StringLiteral = "a"]:
         """Closes the file handle."""
         _ = fclose(self.handle)
 
-    fn readline(self) -> String:
+    fn readline(self) raises -> String:
         """Reads an entire line from stdin or until EOF. Lines are delimited by a newline character.
 
         Returns:
@@ -94,7 +94,7 @@ struct _fdopen[mode: StringLiteral = "a"]:
         """
         return self.read_until_delimiter("\n")
 
-    fn read_until_delimiter(self, delimiter: String) -> String:
+    fn read_until_delimiter(self, delimiter: String) raises -> String:
         """Reads an entire line from a stream, up to the `delimiter`.
         Does not include the delimiter in the result.
 
@@ -139,6 +139,13 @@ struct _fdopen[mode: StringLiteral = "a"]:
             ord(delimiter),
             self.handle,
         )
+        # Per man getdelim(3), getdelim will return -1 if an error occurs
+        # (or the user sends EOF without providing any input). We must
+        # raise an error in this case because otherwise, String() will crash mojo
+        # if the user sends EOF with no input.
+        # TODO: check errno to ensure we haven't encountered EINVAL or ENOMEM instead
+        if bytes_read == -1:
+            raise Error("EOF")
         # Copy the buffer (excluding the delimiter itself) into a Mojo String.
         var s = String(StringRef(buffer, bytes_read - 1))
         # Explicitly free the buffer using free() instead of the Mojo allocator.
@@ -286,7 +293,7 @@ fn print[
 # ===----------------------------------------------------------------------=== #
 
 
-fn input(prompt: String = "") -> String:
+fn input(prompt: String = "") raises -> String:
     """Reads a line of input from the user.
 
     Reads a line from standard input, converts it to a string, and returns that string.

stdlib/test/builtin/test_issue_3908.mojo

@@ -0,0 +1,28 @@
+# ===----------------------------------------------------------------------=== #
+# Copyright (c) 2024, Modular Inc. All rights reserved.
+#
+# Licensed under the Apache License v2.0 with LLVM Exceptions:
+# https://llvm.org/LICENSE.txt
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ===----------------------------------------------------------------------=== #
+# RUN: echo -n | %mojo %s
+
+from builtin.io import _fdopen
+from testing import testing
+
+
+fn test_read_until_delimiter_raises_eof() raises:
+    var stdin = _fdopen["r"](0)
+    with testing.assert_raises(contains="EOF"):
+        # Assign to a variable to silence a warning about unused String value
+        # if an error wasn't raised.
+        var unused = stdin.read_until_delimiter("\n")
+
+
+fn main() raises:
+    test_read_until_delimiter_raises_eof()