sn-fetch PoC: range probe, weighted planner, parallel segments, reassembly + sha256

server/crates/sn-fetch/src/probe.rs

@@ -0,0 +1,66 @@
+//! Probing: find out whether a server supports HTTP range requests and
+//! how large the object is, before planning a segmented download.
+
+use reqwest::header::{ACCEPT_RANGES, CONTENT_LENGTH};
+use reqwest::StatusCode;
+
+use crate::error::FetchError;
+use crate::types::{FileSize, RangeSupport, TargetUrl};
+
+/// What a HEAD request (plus fallback range probe) told us about the target.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct ProbeInfo {
+    pub range_support: RangeSupport,
+    pub content_length: Option<FileSize>,
+}
+
+/// Probe the target: HEAD first; if the `Accept-Ranges` header is absent
+/// or inconclusive, fall back to a 1-byte GET with a `Range` header and
+/// check for `206 Partial Content`.
+pub async fn probe(client: &reqwest::Client, url: &TargetUrl) -> Result<ProbeInfo, FetchError> {
+    let head = client
+        .head(url.inner().clone())
+        .send()
+        .await?
+        .error_for_status()?;
+
+    let content_length = head
+        .headers()
+        .get(CONTENT_LENGTH)
+        .and_then(|v| v.to_str().ok())
+        .and_then(|v| v.parse::<u64>().ok());
+
+    let advertised = match head
+        .headers()
+        .get(ACCEPT_RANGES)
+        .and_then(|v| v.to_str().ok())
+        .map(str::trim)
+    {
+        Some("bytes") => Some(RangeSupport::Supported),
+        Some("none") => Some(RangeSupport::Unsupported),
+        _ => None,
+    };
+
+    let range_support = match advertised {
+        Some(support) => support,
+        // HEAD was inconclusive: ask directly with a 1-byte range.
+        None => {
+            let resp = client
+                .get(url.inner().clone())
+                .header(reqwest::header::RANGE, "bytes=0-0")
+                .send()
+                .await?;
+
+            match resp.status() {
+                StatusCode::PARTIAL_CONTENT => RangeSupport::Supported,
+                StatusCode::OK => RangeSupport::Unsupported,
+                _ => RangeSupport::Unknown,
+            }
+        }
+    };
+
+    Ok(ProbeInfo {
+        range_support,
+        content_length: content_length.and_then(|len| FileSize::try_new(len).ok()),
+    })
+}