When reading from stdin, put a wrapper around the IO object

The purpose of this commit is to fix Bug #21188.  We need to detect when
stdin has run in to an EOF case.  Unfortunately we can't _call_ the eof
function on IO because it will block.

Here is a short script to demonstrate the issue:

```ruby
x = STDIN.gets
puts x
puts x.eof?
```

If you run the script, then type some characters (but _NOT_ a newline),
then hit Ctrl-D twice, it will print the input string.  Unfortunately,
calling `eof?` will try to read from STDIN again causing us to need a
3rd Ctrl-D to exit the program.

Before introducing the EOF callback to Prism, the input loop looked
kind of like this:

```ruby
loop do
  str = STDIN.gets
  process(str)

  if str.nil?
    p :DONE
  end
end
```

Which required 3 Ctrl-D to exit.  If we naively changed it to something
like this:

```ruby
loop do
  str = STDIN.gets
  process(str)

  if STDIN.eof?
    p :DONE
  end
end
```

It would still require 3 Ctrl-D because `eof?` would block.  In this
patch, we're wrapping the IO object, checking the buffer for a newline
and length, and then using that to simulate a non-blocking eof? method.

This commit wraps STDIN and emulates a non-blocking `eof` function.

[Bug #21188]

lib/prism/ffi.rb

@@ -86,6 +86,7 @@ module Prism
     end
 
     callback :pm_parse_stream_fgets_t, [:pointer, :int, :pointer], :pointer
+    callback :pm_parse_stream_feof_t, [:pointer], :int
     enum :pm_string_init_result_t, %i[PM_STRING_INIT_SUCCESS PM_STRING_INIT_ERROR_GENERIC PM_STRING_INIT_ERROR_DIRECTORY]
     enum :pm_string_query_t, [:PM_STRING_QUERY_ERROR, -1, :PM_STRING_QUERY_FALSE, :PM_STRING_QUERY_TRUE]
 
@@ -101,7 +102,7 @@ module Prism
       "pm_string_query_local",
       "pm_string_query_constant",
       "pm_string_query_method_name",
-      [:pm_parse_stream_fgets_t]
+      [:pm_parse_stream_fgets_t, :pm_parse_stream_feof_t]
     )
 
     load_exported_functions_from(
@@ -281,12 +282,14 @@ module Prism
           end
         }
 
+        eof_callback = -> (_) { stream.eof?  }
+
         # In the pm_serialize_parse_stream function it accepts a pointer to the
         # IO object as a void* and then passes it through to the callback as the
         # third argument, but it never touches it itself. As such, since we have
         # access to the IO object already through the closure of the lambda, we
         # can pass a null pointer here and not worry.
-        LibRubyParser.pm_serialize_parse_stream(buffer.pointer, nil, callback, dump_options(options))
+        LibRubyParser.pm_serialize_parse_stream(buffer.pointer, nil, callback, eof_callback, dump_options(options))
         Prism.load(source, buffer.read, options.fetch(:freeze, false))
       end
     end

prism/extension.c

@@ -994,6 +994,14 @@ profile_file(int argc, VALUE *argv, VALUE self) {
     return Qnil;
 }
 
+static int
+parse_stream_eof(void *stream) {
+    if (rb_funcall((VALUE) stream, rb_intern("eof?"), 0)) {
+        return 1;
+    }
+    return 0;
+}
+
 /**
  * An implementation of fgets that is suitable for use with Ruby IO objects.
  */
@@ -1034,7 +1042,7 @@ parse_stream(int argc, VALUE *argv, VALUE self) {
     pm_parser_t parser;
     pm_buffer_t buffer;
 
-    pm_node_t *node = pm_parse_stream(&parser, &buffer, (void *) stream, parse_stream_fgets, &options);
+    pm_node_t *node = pm_parse_stream(&parser, &buffer, (void *) stream, parse_stream_fgets, parse_stream_eof, &options);
     rb_encoding *encoding = rb_enc_find(parser.encoding->name);
 
     VALUE source = pm_source_new(&parser, encoding, options.freeze);

prism/prism.c

@@ -22848,7 +22848,7 @@ pm_parse(pm_parser_t *parser) {
  * otherwise return true.
  */
 static bool
-pm_parse_stream_read(pm_buffer_t *buffer, void *stream, pm_parse_stream_fgets_t *stream_fgets) {
+pm_parse_stream_read(pm_buffer_t *buffer, void *stream, pm_parse_stream_fgets_t *stream_fgets, pm_parse_stream_feof_t *stream_feof) {
 #define LINE_SIZE 4096
     char line[LINE_SIZE];
 
@@ -22884,6 +22884,12 @@ pm_parse_stream_read(pm_buffer_t *buffer, void *stream, pm_parse_stream_fgets_t
                 if (strncmp(line, "__END__\r\n", 9) == 0) return false;
                 break;
         }
+
+        // All data should be read via gets.  If the string returned by gets
+        // _doesn't_ end with a newline, then we assume we hit EOF condition.
+        if (stream_feof(stream)) {
+            break;
+        }
     }
 
     return true;
@@ -22919,16 +22925,17 @@ pm_parse_stream_unterminated_heredoc_p(pm_parser_t *parser) {
  * can stream stdin in to Ruby so we need to support a streaming API.
  */
 PRISM_EXPORTED_FUNCTION pm_node_t *
-pm_parse_stream(pm_parser_t *parser, pm_buffer_t *buffer, void *stream, pm_parse_stream_fgets_t *stream_fgets, const pm_options_t *options) {
+pm_parse_stream(pm_parser_t *parser, pm_buffer_t *buffer, void *stream, pm_parse_stream_fgets_t *stream_fgets, pm_parse_stream_feof_t *stream_feof, const pm_options_t *options) {
     pm_buffer_init(buffer);
 
-    bool eof = pm_parse_stream_read(buffer, stream, stream_fgets);
+    bool eof = pm_parse_stream_read(buffer, stream, stream_fgets, stream_feof);
+
     pm_parser_init(parser, (const uint8_t *) pm_buffer_value(buffer), pm_buffer_length(buffer), options);
     pm_node_t *node = pm_parse(parser);
 
     while (!eof && parser->error_list.size > 0 && (parser->lex_modes.index > 0 || pm_parse_stream_unterminated_heredoc_p(parser))) {
         pm_node_destroy(parser, node);
-        eof = pm_parse_stream_read(buffer, stream, stream_fgets);
+        eof = pm_parse_stream_read(buffer, stream, stream_fgets, stream_feof);
 
         pm_parser_free(parser);
         pm_parser_init(parser, (const uint8_t *) pm_buffer_value(buffer), pm_buffer_length(buffer), options);
@@ -23020,13 +23027,13 @@ pm_serialize_parse(pm_buffer_t *buffer, const uint8_t *source, size_t size, cons
  * given stream into to the given buffer.
  */
 PRISM_EXPORTED_FUNCTION void
-pm_serialize_parse_stream(pm_buffer_t *buffer, void *stream, pm_parse_stream_fgets_t *stream_fgets, const char *data) {
+pm_serialize_parse_stream(pm_buffer_t *buffer, void *stream, pm_parse_stream_fgets_t *stream_fgets, pm_parse_stream_feof_t *stream_feof, const char *data) {
     pm_parser_t parser;
     pm_options_t options = { 0 };
     pm_options_read(&options, data);
 
     pm_buffer_t parser_buffer;
-    pm_node_t *node = pm_parse_stream(&parser, &parser_buffer, stream, stream_fgets, &options);
+    pm_node_t *node = pm_parse_stream(&parser, &parser_buffer, stream, stream_fgets, stream_feof, &options);
     pm_serialize_header(buffer);
     pm_serialize_content(&parser, node, buffer);
     pm_buffer_append_byte(buffer, '\0');

prism/prism.h

@@ -87,6 +87,13 @@ PRISM_EXPORTED_FUNCTION pm_node_t * pm_parse(pm_parser_t *parser);
  */
 typedef char * (pm_parse_stream_fgets_t)(char *string, int size, void *stream);
 
+/**
+ * This function is used in pm_parse_stream to check whether a stream is EOF.
+ * It closely mirrors that of feof so that feof can be used as the
+ * default implementation.
+ */
+typedef int (pm_parse_stream_feof_t)(void *stream);
+
 /**
  * Parse a stream of Ruby source and return the tree.
  *
@@ -94,10 +101,11 @@ typedef char * (pm_parse_stream_fgets_t)(char *string, int size, void *stream);
  * @param buffer The buffer to use.
  * @param stream The stream to parse.
  * @param stream_fgets The function to use to read from the stream.
+ * @param stream_feof The function to use to determine if the stream has hit eof.
  * @param options The optional options to use when parsing.
  * @return The AST representing the source.
  */
-PRISM_EXPORTED_FUNCTION pm_node_t * pm_parse_stream(pm_parser_t *parser, pm_buffer_t *buffer, void *stream, pm_parse_stream_fgets_t *stream_fgets, const pm_options_t *options);
+PRISM_EXPORTED_FUNCTION pm_node_t * pm_parse_stream(pm_parser_t *parser, pm_buffer_t *buffer, void *stream, pm_parse_stream_fgets_t *stream_fgets, pm_parse_stream_feof_t *stream_feof, const pm_options_t *options);
 
 // We optionally support serializing to a binary string. For systems that don't
 // want or need this functionality, it can be turned off with the
@@ -111,9 +119,10 @@ PRISM_EXPORTED_FUNCTION pm_node_t * pm_parse_stream(pm_parser_t *parser, pm_buff
  * @param buffer The buffer to serialize to.
  * @param stream The stream to parse.
  * @param stream_fgets The function to use to read from the stream.
+ * @param stream_feof The function to use to tell if the stream has hit eof.
  * @param data The optional data to pass to the parser.
  */
-PRISM_EXPORTED_FUNCTION void pm_serialize_parse_stream(pm_buffer_t *buffer, void *stream, pm_parse_stream_fgets_t *stream_fgets, const char *data);
+PRISM_EXPORTED_FUNCTION void pm_serialize_parse_stream(pm_buffer_t *buffer, void *stream, pm_parse_stream_fgets_t *stream_fgets, pm_parse_stream_feof_t *stream_feof, const char *data);
 
 /**
  * Serialize the given list of comments to the given buffer.

prism_compile.c

@@ -11492,6 +11492,18 @@ pm_parse_string(pm_parse_result_t *result, VALUE source, VALUE filepath, VALUE *
     return pm_parse_process(result, node, script_lines);
 }
 
+struct rb_stdin_wrapper {
+    VALUE rb_stdin;
+    int eof_seen;
+};
+
+static int
+pm_parse_stdin_eof(void *stream)
+{
+    struct rb_stdin_wrapper * wrapped_stdin = (struct rb_stdin_wrapper *)stream;
+    return wrapped_stdin->eof_seen;
+}
+
 /**
  * An implementation of fgets that is suitable for use with Ruby IO objects.
  */
@@ -11500,7 +11512,9 @@ pm_parse_stdin_fgets(char *string, int size, void *stream)
 {
     RUBY_ASSERT(size > 0);
 
-    VALUE line = rb_funcall((VALUE) stream, rb_intern("gets"), 1, INT2FIX(size - 1));
+    struct rb_stdin_wrapper * wrapped_stdin = (struct rb_stdin_wrapper *)stream;
+
+    VALUE line = rb_funcall(wrapped_stdin->rb_stdin, rb_intern("gets"), 1, INT2FIX(size - 1));
     if (NIL_P(line)) {
         return NULL;
     }
@@ -11511,6 +11525,13 @@ pm_parse_stdin_fgets(char *string, int size, void *stream)
     memcpy(string, cstr, length);
     string[length] = '\0';
 
+    // We're reading strings from stdin via gets.  We'll assume that if the
+    // string is smaller than the requested length, and doesn't end with a
+    // newline, that we hit EOF.
+    if (length < (size - 1) && string[length - 1] != '\n') {
+        wrapped_stdin->eof_seen = 1;
+    }
+
     return string;
 }
 
@@ -11527,8 +11548,13 @@ pm_parse_stdin(pm_parse_result_t *result)
 {
     pm_options_frozen_string_literal_init(&result->options);
 
+    struct rb_stdin_wrapper wrapped_stdin = {
+        rb_stdin,
+        0
+    };
+
     pm_buffer_t buffer;
-    pm_node_t *node = pm_parse_stream(&result->parser, &buffer, (void *) rb_stdin, pm_parse_stdin_fgets, &result->options);
+    pm_node_t *node = pm_parse_stream(&result->parser, &buffer, (void *) &wrapped_stdin, pm_parse_stdin_fgets, pm_parse_stdin_eof, &result->options);
 
     // Copy the allocated buffer contents into the input string so that it gets
     // freed. At this point we've handed over ownership, so we don't need to