Allow checking HTTP version support at runtime (#368)

Add `is_http_version_supported` which allows you to check which HTTP versions are supported by a particular build of Isahc at runtime. This could be used to decide whether an application would choose to enforce HTTP/2 for example, or just to verify that build configuration is as expected.

build.rs

@@ -1,8 +1,7 @@
-use std::{env, error::Error, fs, path::PathBuf};
+use std::{env, error::Error};
 
 fn main() -> Result<(), Box<dyn Error>> {
-    let out_dir = PathBuf::from(env::var("OUT_DIR")?);
-    fs::write(out_dir.join("features.txt"), get_feature_string())?;
+    println!("cargo:rustc-env=ISAHC_FEATURES={}", get_feature_string());
 
     Ok(())
 }
@@ -13,6 +12,7 @@ fn get_feature_string() -> String {
         .filter(|(name, _)| name.starts_with("CARGO_FEATURE_"))
         .filter(|(_, value)| value == "1")
         .map(|(name, _)| name.trim_start_matches("CARGO_FEATURE_").to_lowercase())
+        .map(|name| name.replace('_', "-"))
         .collect::<Vec<String>>()
         .join(",")
 }

src/info.rs

@@ -0,0 +1,89 @@
+//! Runtime support for checking versions and feature availability.
+
+use once_cell::sync::Lazy;
+
+// Query for curl version info just once since it is immutable.
+static CURL_VERSION: Lazy<curl::Version> = Lazy::new(curl::Version::get);
+
+/// Gets a human-readable string with the version number of Isahc and its
+/// dependencies.
+///
+/// This function can be helpful when troubleshooting issues in Isahc or one of
+/// its dependencies.
+pub fn version() -> &'static str {
+    static VERSION_STRING: Lazy<String> = Lazy::new(|| {
+        format!(
+            "isahc/{} (features:{}) {}",
+            env!("CARGO_PKG_VERSION"),
+            env!("ISAHC_FEATURES"),
+            curl::Version::num(),
+        )
+    });
+
+    &VERSION_STRING
+}
+
+/// Check if runtime support is available for the given HTTP version.
+///
+/// This only indicates whether support for communicating with this HTTP version
+/// is available, which is usually determined by which features were enabled
+/// during compilation, but can also be affected by what is available in system
+/// libraries when using dynamic linking.
+///
+/// This does not indicate which versions Isahc will attempt to use by default.
+/// To customize which versions to use within a particular client or request
+/// instance, see [`VersionNegotiation`][crate::config::VersionNegotiation].
+pub fn is_http_version_supported(version: http::Version) -> bool {
+    match version {
+        // HTTP/0.9 was disabled by default as of 7.66.0. See also
+        // https://github.com/sagebind/isahc/issues/310 if we ever decide to
+        // allow enabling it again.
+        http::Version::HTTP_09 => match curl_version() {
+            (7, minor, _) if minor < 66 => true,
+            (major, _, _) if major < 7 => true,
+            _ => false,
+        },
+        http::Version::HTTP_10 => true,
+        http::Version::HTTP_11 => true,
+        http::Version::HTTP_2 => CURL_VERSION.feature_http2(),
+        http::Version::HTTP_3 => CURL_VERSION.feature_http3(),
+        _ => false,
+    }
+}
+
+fn curl_version() -> (u8, u8, u8) {
+    let bits = CURL_VERSION.version_num();
+
+    ((bits >> 16) as u8, (bits >> 8) as u8, bits as u8)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn version_expected() {
+        let version = version();
+
+        assert!(version.starts_with("isahc/1."));
+        assert!(version.contains("curl/7."));
+    }
+
+    #[test]
+    fn curl_version_expected() {
+        let (major, minor, _patch) = curl_version();
+
+        assert_eq!(major, 7);
+        assert!(minor > 0);
+    }
+
+    #[test]
+    fn http1_always_supported() {
+        assert!(is_http_version_supported(http::Version::HTTP_10));
+        assert!(is_http_version_supported(http::Version::HTTP_11));
+
+        if cfg!(feature = "http2") {
+            assert!(is_http_version_supported(http::Version::HTTP_2));
+        }
+    }
+}

src/lib.rs

@@ -160,6 +160,9 @@
 //! makes it available. To configure which HTTP versions to use in a request,
 //! see [`VersionNegotiation`](config::VersionNegotiation).
 //!
+//! To check which HTTP versions are supported at runtime, you can use
+//! [`is_http_version_supported`].
+//!
 //! Enabled by default.
 //!
 //! ## `json`
@@ -239,7 +242,6 @@
 // These lints suggest to use features not available in our MSRV.
 #![allow(clippy::manual_strip, clippy::match_like_matches_macro)]
 
-use once_cell::sync::Lazy;
 use std::convert::TryFrom;
 
 #[macro_use]
@@ -254,6 +256,7 @@ mod client;
 mod default_headers;
 mod handler;
 mod headers;
+mod info;
 mod metrics;
 mod parsing;
 mod redirect;
@@ -278,6 +281,7 @@ pub use crate::{
     client::{HttpClient, HttpClientBuilder, ResponseFuture},
     error::Error,
     http::{request::Request, response::Response},
+    info::*,
     metrics::Metrics,
     request::RequestExt,
     response::{AsyncReadResponseExt, ReadResponseExt, ResponseExt},
@@ -469,22 +473,3 @@ pub fn send<B: Into<Body>>(request: Request<B>) -> Result<Response<Body>, Error>
 pub fn send_async<B: Into<AsyncBody>>(request: Request<B>) -> ResponseFuture<'static> {
     HttpClient::shared().send_async(request)
 }
-
-/// Gets a human-readable string with the version number of Isahc and its
-/// dependencies.
-///
-/// This function can be helpful when troubleshooting issues in Isahc or one of
-/// its dependencies.
-pub fn version() -> &'static str {
-    static FEATURES_STRING: &str = include_str!(concat!(env!("OUT_DIR"), "/features.txt"));
-    static VERSION_STRING: Lazy<String> = Lazy::new(|| {
-        format!(
-            "isahc/{} (features:{}) {}",
-            env!("CARGO_PKG_VERSION"),
-            FEATURES_STRING,
-            curl::Version::num(),
-        )
-    });
-
-    &VERSION_STRING
-}