feat(profiling): FallibleStringWriter

libdd-profiling/src/profiles/fallible_string_writer.rs

@@ -0,0 +1,187 @@
+// Copyright 2025-Present Datadog, Inc. https://www.datadoghq.com/
+// SPDX-License-Identifier: Apache-2.0
+
+use core::fmt::{self, Write};
+use std::collections::TryReserveError;
+
+/// A `fmt::Write` adapter that grows a `String` using `try_reserve` before
+/// each write, returning `fmt::Error` on allocation failure.
+#[derive(Debug)]
+pub struct FallibleStringWriter {
+    buf: String,
+}
+
+impl Default for FallibleStringWriter {
+    fn default() -> FallibleStringWriter {
+        FallibleStringWriter::new()
+    }
+}
+
+impl FallibleStringWriter {
+    /// Creates a new empty string writer.
+    pub const fn new() -> Self {
+        Self { buf: String::new() }
+    }
+
+    /// Creates a new fallible string writer with a previously existing string
+    /// as the start of the buffer. New writes will append to the end of this.
+    pub const fn new_from_existing(buf: String) -> FallibleStringWriter {
+        FallibleStringWriter { buf }
+    }
+
+    /// Tries to reserve capacity for at least additional bytes more than the
+    /// current length. The allocator may reserve more space to speculatively
+    /// avoid frequent allocations.
+    pub fn try_reserve(&mut self, len: usize) -> Result<(), TryReserveError> {
+        self.buf.try_reserve(len)
+    }
+
+    /// Tries to reserve the minimum capacity for at least `additional` bytes
+    /// more than the current length. Unlike [`try_reserve`], this will not
+    /// deliberately over-allocate to speculatively avoid frequent allocations.
+    ///
+    /// Note that the allocator may give the collection more space than it
+    /// requests. Therefore, capacity can not be relied upon to be precisely
+    /// minimal. Prefer [`try_reserve`] if future insertions are expected.
+    pub fn try_reserve_exact(&mut self, len: usize) -> Result<(), TryReserveError> {
+        self.buf.try_reserve_exact(len)
+    }
+
+    pub fn try_push_str(&mut self, str: &str) -> Result<(), TryReserveError> {
+        self.try_reserve(str.len())?;
+        self.buf.push_str(str);
+        Ok(())
+    }
+}
+
+impl From<FallibleStringWriter> for String {
+    fn from(w: FallibleStringWriter) -> String {
+        w.buf
+    }
+}
+
+impl From<String> for FallibleStringWriter {
+    fn from(buf: String) -> FallibleStringWriter {
+        FallibleStringWriter { buf }
+    }
+}
+
+impl Write for FallibleStringWriter {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        self.try_push_str(s).map_err(|_| fmt::Error)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fmt::Write;
+
+    #[test]
+    fn test_new_and_default() {
+        let writer = FallibleStringWriter::new();
+        let s: String = writer.into();
+        assert_eq!(s, "");
+
+        let writer = FallibleStringWriter::default();
+        let s: String = writer.into();
+        assert_eq!(s, "");
+    }
+
+    #[test]
+    fn test_write_str() {
+        let mut writer = FallibleStringWriter::new();
+        writer.write_str("Hello").unwrap();
+        writer.write_str(", ").unwrap();
+        writer.write_str("World!").unwrap();
+
+        let s: String = writer.into();
+        assert_eq!(s, "Hello, World!");
+    }
+
+    #[test]
+    fn test_write_formatted() {
+        let mut writer = FallibleStringWriter::new();
+        write!(writer, "x = {}, ", 10).unwrap();
+        write!(writer, "y = {}, ", 20).unwrap();
+        write!(writer, "sum = {}", 10 + 20).unwrap();
+
+        let s: String = writer.into();
+        assert_eq!(s, "x = 10, y = 20, sum = 30");
+    }
+
+    #[test]
+    fn test_try_push_str() {
+        let mut writer = FallibleStringWriter::new();
+        writer.try_push_str("Hello").unwrap();
+        writer.try_push_str(" ").unwrap();
+        writer.try_push_str("World").unwrap();
+
+        let s: String = writer.into();
+        assert_eq!(s, "Hello World");
+    }
+
+    #[test]
+    fn test_try_reserve() {
+        // Marcus Aurelius, Meditations (public domain)
+        let strings = [
+            "The happiness of your life depends upon the quality of your thoughts: ",
+            "therefore, guard accordingly, and take care that you entertain ",
+            "no notions unsuitable to virtue and reasonable nature.",
+        ];
+        let total_len: usize = strings.iter().map(|s| s.len()).sum();
+
+        let mut writer = FallibleStringWriter::new();
+        // Asking for more than is needed just to ensure that the test isn't
+        // accidentally correct.
+        let capacity = 2 * total_len + 7;
+        writer.try_reserve_exact(capacity).unwrap();
+
+        // After reserving, we should be able to write all strings (and more).
+        for s in &strings {
+            writer.write_str(s).unwrap();
+        }
+
+        let result: String = writer.into();
+        assert_eq!(result, strings.join(""));
+
+        // It can't be less, but an allocator is free to round, even on a
+        // try_reserve_exact.
+        assert!(result.capacity() >= capacity);
+    }
+
+    #[test]
+    fn test_from_existing_string() {
+        // Test From<String>, new_from_existing, and appending
+        let s = String::from("start: ");
+        let mut writer = FallibleStringWriter::from(s);
+        write!(writer, "{}", 123).unwrap();
+        assert_eq!(String::from(writer), "start: 123");
+
+        // Test new_from_existing
+        let mut writer = FallibleStringWriter::new_from_existing(String::from("prefix-"));
+        writer.try_push_str("suffix").unwrap();
+        assert_eq!(String::from(writer), "prefix-suffix");
+    }
+
+    #[test]
+    fn test_write_unicode() {
+        let mut writer = FallibleStringWriter::new();
+        write!(writer, "Hello 👋 World 🌍").unwrap();
+
+        let s: String = writer.into();
+        assert_eq!(s, "Hello 👋 World 🌍");
+    }
+
+    #[test]
+    fn test_write_long_string() {
+        let mut writer = FallibleStringWriter::new();
+        let long_str = "a".repeat(1000);
+
+        writer.write_str(&long_str).unwrap();
+
+        let s: String = writer.into();
+        assert_eq!(s.len(), 1000);
+        assert_eq!(s, long_str);
+    }
+}

libdd-profiling/src/profiles/mod.rs

@@ -4,5 +4,7 @@
 pub mod collections;
 mod compressor;
 pub mod datatypes;
+mod fallible_string_writer;
 
 pub use compressor::*;
+pub use fallible_string_writer::*;