⚡ Bolt: Optimize percent_decode to avoid allocation on unencoded strings - #424
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This avoids allocating strings entirely when the input text does not contain
any url-encoded characters ('%' or '+'). By checking first and returning
Cow::Borrowed on the fast path, string processing time is reduced significantly.
Calls in parse_key_value_pairs are updated to handle Cow via .into_owned()
and .as_ref().
Co-authored-by: logbie <1138960+logbie@users.noreply.github.com>
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Pull request overview
Optimizes URL percent-decoding in the stdlib text utilities to reduce allocations on the hot path used by query/form/cookie parsing.
Changes:
- Updated
percent_decodeto returnCow<'_, str>with a borrowed fast-path when no%/+is present. - Pre-allocated decode buffer capacity for encoded inputs.
- Adjusted
parse_key_value_pairscall sites to accommodate the new return type.
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| let decoded_key = percent_decode(key).into_owned(); | ||
| let decoded_value = percent_decode(value); | ||
| params.insert(decoded_key, Value::Text(Arc::from(decoded_value))); | ||
| params.insert(decoded_key, Value::Text(Arc::from(decoded_value.as_ref()))); |
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decoded_value is a Cow<str>, but converting it with Arc::from(decoded_value.as_ref()) forces an extra allocation+copy for the Cow::Owned case (the owned String gets dropped after being copied into a new Arc<str>). Prefer converting in a way that reuses the owned String buffer when available (e.g., branch on the Cow and use Arc::from(String) for Owned).
| params.insert(decoded_key, Value::Text(Arc::from(decoded_value.as_ref()))); | |
| let value_arc: Arc<str> = match decoded_value { | |
| Cow::Borrowed(s) => Arc::from(s), | |
| Cow::Owned(s) => Arc::from(s), | |
| }; | |
| params.insert(decoded_key, Value::Text(value_arc)); |
| fn percent_decode(s: &str) -> Cow<'_, str> { | ||
| let bytes = s.as_bytes(); | ||
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| // Optimization: avoid string allocation and decoding overhead if the string | ||
| // doesn't contain any encoded characters ('%' or '+'). | ||
| // We scan bytes in a single pass to be efficient. | ||
| if !bytes.iter().any(|&b| b == b'%' || b == b'+') { | ||
| return Cow::Borrowed(s); | ||
| } |
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This changes the return type/behavioral path of URL decoding (borrowed fast-path vs owned decoding) but there are no Rust tests covering parse_query_string/parse_key_value_pairs behavior (including '+' handling and invalid % sequences). Since other text stdlib features are covered by integration tests, adding coverage here would help prevent regressions when optimizing this hot path.
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🧹 Nitpick comments (1)
src/stdlib/text.rs (1)
16-58: Add regression tests for new decode branches.Please add tests that explicitly cover: (1) no
%/+input path, (2) malformed%sequences, and (3) mixed encoded/unencoded inputs. This will lock in the new branching behavior.🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed. In `@src/stdlib/text.rs` around lines 16 - 58, Add unit tests for percent_decode to cover the three branches: (1) input without '%' or '+' should return a borrowed Cow::Borrowed string (test with a simple ASCII string), (2) malformed '%' sequences should preserve the '%' and not panic (test cases like "%", "%G", "%1" and ensure output contains the literal '%' and other bytes handled as in the implementation), and (3) mixed encoded/unencoded inputs should decode percent-hex pairs and '+' to space while leaving other bytes intact (e.g., "a+%20b%41c" -> verify resulting string). Place tests alongside other text module tests and call percent_decode directly to assert Cow variant and output contents.
🤖 Prompt for all review comments with AI agents
Verify each finding against the current code and only fix it if needed.
Nitpick comments:
In `@src/stdlib/text.rs`:
- Around line 16-58: Add unit tests for percent_decode to cover the three
branches: (1) input without '%' or '+' should return a borrowed Cow::Borrowed
string (test with a simple ASCII string), (2) malformed '%' sequences should
preserve the '%' and not panic (test cases like "%", "%G", "%1" and ensure
output contains the literal '%' and other bytes handled as in the
implementation), and (3) mixed encoded/unencoded inputs should decode
percent-hex pairs and '+' to space while leaving other bytes intact (e.g.,
"a+%20b%41c" -> verify resulting string). Place tests alongside other text
module tests and call percent_decode directly to assert Cow variant and output
contents.
💡 What:
Optimized
percent_decodeinsrc/stdlib/text.rsto returnstd::borrow::Cow<'_, str>instead of unconditionally creating and returning an ownedString.Added a fast path check that scans the string bytes to see if it contains any
%or+characters. If it doesn't, it returns aCow::Borrowed.🎯 Why:
The
percent_decodefunction is called heavily insideparse_key_value_pairs(used for parsing query parameters, forms, and cookies). In most cases, the strings being parsed do not contain percent-encoded characters, meaning the constant allocation ofVec<u8>and conversion intoStringcauses unnecessary performance overhead.📊 Impact:
According to benchmarks:
Vec::with_capacity).🔬 Measurement:
Run the standard test suite:
cargo testandcargo bench(if configured), or observe the performance improvement when processing high volumes of incoming HTTP requests with query strings.PR created automatically by Jules for task 1686376902912650404 started by @logbie
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