Backport(v1.19): config: accept empty lines in quoted strings (#5448) (#5478)

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**Which issue(s) this PR fixes**: 
Backport #5448
Fixes #4691

**What this PR does / why we need it**: 

The quoted configuration values previously handled a physical line
ending together with its preceding character. When the parser was
already positioned at a line ending—such as on an empty line or after an
escaped literal backslash—it instead reported an unexpected end of file.

Handle backslash line continuations and preserved line endings
explicitly. This accepts empty lines, preserves LF and CRLF, and retains
the existing single-backslash continuation behavior.

**Docs Changes**:

None. The existing multiline-string documentation already describes the
intended behavior.

**Release Note**: 

config: accept empty lines in quoted strings.

**Testing**:

Automated:

- `bundle exec rake test TEST=test/config/test_literal_parser.rb` — 220
tests, 223 assertions, 0 failures, 0 errors
- `TEST_ENV_NUMBER=focus4691 bundle exec rake test
TEST=test/config/test_config_parser.rb` — 56 tests, 106 assertions, 0
failures, 0 errors
- `bundle exec rake test` (two independent runs) — 4,343 tests, 15,829
and 15,830 assertions respectively; 0 failures, 0 errors, 3 pendings,
and 36 omissions in both
- `rubocop` — 459 files inspected, no offenses
- `ruby -c` for all three changed Ruby files and `git diff --check`

Manual (Ruby 4.0.6):

- Built and separately installed gems from baseline
`f380d996bc828b5cd488b578018c06826ebcbf7e` and candidate
`7d017869a55e179da4d195806f26d33dc485fc9d`.
- The baseline built gem rejected the reproducing configuration with a
false unexpected-EOF error; the candidate built gem accepted it with
`fluentd --dry-run`; a genuinely unterminated double-quoted value
remained rejected.
- Ran the candidate built gem as a separate `fluentd --no-supervisor`
process through `dummy` → `record_transformer` → `stdout` and externally
parsed its emitted JSON as
`{"message":"seed","blank":"world\n\n","literal_backslash":"left\\\nright","continued":"leftright"}`.

Signed-off-by: Akash Kumar <meakash7902@gmail.com>
Signed-off-by: Shizuo Fujita <fujita@clear-code.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Akash Kumar <91385321+AkashKumar7902@users.noreply.github.com>
Co-authored-by: Shizuo Fujita <fujita@clear-code.com>

Author: 3 people

lib/fluent/config/basic_parser.rb

@@ -28,7 +28,6 @@ def initialize(strscan)
       SPACING = /(?:[ \t\r\n]|\z|\#.*?(?:\z|[\r\n]))+/
       ZERO_OR_MORE_SPACING = /(?:[ \t\r\n]|\z|\#.*?(?:\z|[\r\n]))*/
       SPACING_WITHOUT_COMMENT = /(?:[ \t\r\n]|\z)+/
-      LINE_END_WITHOUT_SPACING_AND_COMMENT = /(?:\z|[\r\n])/
 
       module ClassMethods
         def symbol(string)

lib/fluent/config/literal_parser.rb

@@ -27,6 +27,15 @@
 module Fluent
   module Config
     class LiteralParser < BasicParser
+      # A physical line break (LF, CR, or CRLF) inside a quoted string.
+      # CRLF is normalized to LF so that the same config text does not produce a
+      # different value depending on whether the file was saved with LF or CRLF.
+      # A lone CR is kept as-is, and an escaped "\r\n" still produces CRLF.
+      LINE_BREAK = /\r\n|[\r\n]/
+      # A backslash immediately followed by a physical line break.
+      # It works as a line continuation, so both are stripped from the value.
+      LINE_CONTINUATION = /\\#{LINE_BREAK}/o
+
       def self.unescape_char(c)
         case c
         when '"'
@@ -99,11 +108,10 @@ def scan_double_quoted_string
             else
               return string.join
             end
-          elsif check(/[^"]#{LINE_END_WITHOUT_SPACING_AND_COMMENT}/o)
-            if s = check(/[^\\]#{LINE_END_WITHOUT_SPACING_AND_COMMENT}/o)
-              string << s
-            end
-            skip(/[^"]#{LINE_END_WITHOUT_SPACING_AND_COMMENT}/o)
+          elsif skip(LINE_CONTINUATION)
+            next
+          elsif s = scan(LINE_BREAK)
+            string << (s == "\r\n" ? "\n" : s)
           elsif s = scan(/\\./)
             string << eval_escape_char(s[1,1])
           elsif skip(/\#\{/)
@@ -126,6 +134,8 @@ def scan_single_quoted_string
             string << "'"
           elsif s = scan(/\\\\/)
             string << "\\"
+          elsif s = scan(LINE_BREAK)
+            string << (s == "\r\n" ? "\n" : s)
           elsif s = scan(/./)
             string << s
           else

test/config/test_config_parser.rb

@@ -149,6 +149,7 @@ def parse_text(text)
         end
 
         test "support multiline string" do
+          assert_text_parsed_as(e('ROOT', '', {"k1" => "world\n\n"}), "k1 \"world\n\n\"")
           assert_text_parsed_as(e('ROOT', '',
             {"k1" => %[line1
                        line2]

test/config/test_literal_parser.rb

@@ -111,6 +111,14 @@ def test_falseX
       test('"t') { assert_parse_error('"t') }  # non-terminated quoted character
       test("\"t\nt\"") { assert_text_parsed_as("t\nt", "\"t\nt\"" ) } # multiline string
       test("\"t\\\nt\"") { assert_text_parsed_as("tt", "\"t\\\nt\"" ) } # multiline string
+      test("\"t\n\nt\"") { assert_text_parsed_as("t\n\nt", "\"t\n\nt\"") }
+      test("\"\nt\"") { assert_text_parsed_as("\nt", "\"\nt\"") }
+      test("\"t\\\\\nt\"") { assert_text_parsed_as("t\\\nt", "\"t\\\\\nt\"") }
+      test("\"t\r\nt\"") { assert_text_parsed_as("t\nt", "\"t\r\nt\"") }
+      test("\"t\rt\"") { assert_text_parsed_as("t\rt", "\"t\rt\"") }
+      test("\"t\\\r\nt\"") { assert_text_parsed_as("tt", "\"t\\\r\nt\"") }
+      test("\"t\n") { assert_parse_error("\"t\n") }
+      test("\"t\\\n") { assert_parse_error("\"t\\\n") }
       test('t"') { assert_text_parsed_as('t"', 't"') }
       test('"."') { assert_text_parsed_as('.', '"."') }
       test('"*"') { assert_text_parsed_as('*', '"*"') }
@@ -138,6 +146,13 @@ def test_falseX
       test("'\\0'") { assert_text_parsed_as('\0', "'\\0'") }
       test("'\\1'") { assert_text_parsed_as('\1', "'\\1'") }
       test("'t") { assert_parse_error("'t") }  # non-terminated quoted character
+      test("'t\nt'") { assert_text_parsed_as("t\nt", "'t\nt'") }
+      test("'t\n\nt'") { assert_text_parsed_as("t\n\nt", "'t\n\nt'") }
+      test("'\nt'") { assert_text_parsed_as("\nt", "'\nt'") }
+      test("'t\r\nt'") { assert_text_parsed_as("t\nt", "'t\r\nt'") }
+      test("'t\rt'") { assert_text_parsed_as("t\rt", "'t\rt'") }
+      test("'t\\\nt'") { assert_text_parsed_as("t\\\nt", "'t\\\nt'") }
+      test("'t\n") { assert_parse_error("'t\n") }
       test("t'") { assert_text_parsed_as("t'", "t'") }
       test("'.'") { assert_text_parsed_as('.', "'.'") }
       test("'*'") { assert_text_parsed_as('*', "'*'") }