Fix frozen string literal warning in magic detection #123
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Contributor
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Tests pass on my machine and GH Actions, the Buildkite build erroring out seems unrelated and not fixable from this PR. |
simonbaird
reviewed
May 31, 2025
| io.binmode if io.respond_to?(:binmode) | ||
| io.set_encoding(Encoding::BINARY) if io.respond_to?(:set_encoding) | ||
| buffer = "".encode(Encoding::BINARY) | ||
| buffer = (+"").encode(Encoding::BINARY) |
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Perhaps this is better for readability:
Suggested change
| buffer = (+"").encode(Encoding::BINARY) | |
| buffer = String.new.encode(Encoding::BINARY) |
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I see from the bench-marking that you did already consider that option. Also this is more succinct hence better.
Suggested change
| buffer = (+"").encode(Encoding::BINARY) | |
| buffer = String.new(encoding: Encoding::BINARY) |
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I think what you want here is to declare a mutable string with binary encoding. That's
String.new(encoding: Encoding::BINARY)
Contributor
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I’m not sure whether this actually fixes the issue I was trying to fix — tried to bundle install marcel from my repo and it still appeared — but this should still be a slight improvement |
simonbaird
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Jun 2, 2025
As mentioned in the comments, rails/marcel#123 PR should make this go away. This is just for cosmetic reasons. The warning makes the test suite output look untidy.
sheldonh
reviewed
Jul 19, 2025
| io.binmode if io.respond_to?(:binmode) | ||
| io.set_encoding(Encoding::BINARY) if io.respond_to?(:set_encoding) | ||
| buffer = "".encode(Encoding::BINARY) | ||
| buffer = (+"").encode(Encoding::BINARY) |
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Suggested change
| buffer = (+"").encode(Encoding::BINARY) | |
| buffer = String.new(encoding: Encoding::BINARY) |
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Hey, can we try to get this merged in? A lot of our tests are failing on ruby 3.4.4 because of this. And instead of recording deprecations, I would like this to be fixed. Let me know if I can help in any way |
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It’d be great to get this fixed. I’m seeing a lot of deprecation warnings while running my tests. 
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Thanks for merging this and releasing a new version. I’m still seeing this warning in marcel-1.1.0: 
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still getting warnings even after this patch |
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Bumping this as there's not been a response to the "still broken" comments after merge and they may have been missed. I'm still seeing these with 1.1.0 too: |
andynu
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Oct 31, 2025
PR rails#123 fixed frozen string warnings on line 127 (buffer creation) but missed the warnings triggered by calling binmode/set_encoding on StringIO objects at lines 123-124. StringIO is always binary-capable and doesn't need these method calls. Calling them triggers Ruby 3.4 deprecation warnings when running with verbose warnings (-W2), which Rails test suites use by default. This change skips binmode/set_encoding for StringIO objects while maintaining the existing defensive respond_to? checks for File objects.
This was referenced Oct 31, 2025
andynu
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Oct 31, 2025
PR rails#123 fixed frozen string warnings on line 127 (buffer creation) but missed the warnings triggered by calling binmode/set_encoding on StringIO objects at lines 123-124. These methods modify StringIO's internal string encoding, which triggers Ruby 3.4 deprecation warnings when running with verbose warnings (-W2), which Rails test suites use by default. Skip these calls for StringIO objects since marcel's byte-reading operations work correctly without the encoding modification, as demonstrated by all 384 tests passing. This maintains the existing defensive respond_to? checks for File objects.
andynu
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Oct 31, 2025
PR rails#123 fixed frozen string warnings on line 127 (buffer creation) but missed the warnings triggered by calling binmode/set_encoding on StringIO objects at lines 120-121. These methods modify StringIO's internal string encoding, which triggers Ruby 3.4 deprecation warnings when running with verbose warnings (-W2), which Rails test suites use by default. Skip these calls for StringIO objects since marcel's byte-reading operations work correctly without the encoding modification, as demonstrated by all 384 tests passing. This maintains the existing defensive respond_to? checks for File objects.
andynu
added a commit
to andynu/marcel
that referenced
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Oct 31, 2025
PR rails#123 fixed frozen string warnings on line 127 (buffer creation) but missed the warnings triggered by calling binmode/set_encoding on StringIO objects. These methods modify StringIO's internal string encoding, which triggers Ruby 3.4 deprecation warnings when running with verbose warnings (-W2), which Rails test suites use by default. Solution: Make StringIO readonly before encoding modifications by calling close_write. This prevents the internal string modification that triggers the warning, while still allowing binmode/set_encoding to execute normally. Only affects StringIO objects (which have closed_write? method) - File objects are unaffected. This workaround should only be needed until Ruby 3.5, which will include an upstream fix: https://redmine.ruby-lang.org/issues/21280
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With latest Ruby 3.4.x, using marcel emits the following warning:
lib/ruby/gems/3.4.0/gems/marcel-1.0.4/lib/marcel/magic.rb:120: warning: literal string will be frozen in the futureThis occurs because the code was creating an empty string literal used as mutable buffer for I/O operations.
This pull request replaces
"".dup.encode(Encoding::BINARY)with(+"").encode(Encoding::BINARY)in the buffer creation. Using the unary plus operator to explicitly create a mutable string benchmarked slightly faster than.dup.encode()on my local machine.Benchmark Results
String Creation Performance (1M iterations)
"".dup.encode()(+"").encode()String.new(encoding:)Memory Allocation (100K iterations)
"".dup.encode()(+"").encode()String.new(encoding:)Real-world Usage (10K magic detection operations)
"".dup.encode()(+"").encode()String.new(encoding:)