backtracking: Limit backtracking via env var

Use RUSTUP_BACKTRACK_LIMIT (default 60) to limit the backtrack
attempts when trying to find a valid nightly to install.

Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>

README.md

@@ -637,6 +637,10 @@ Command                                                     | Description
 - `RUSTUP_UNPACK_RAM` *unstable* (default 400M, min 100M)
   Caps the amount of RAM rustup will use for IO tasks while unpacking.
 
+- `RUSTUP_BACKTRACK_LIMIT` *unstable* (default 60, min 1)
+  Caps the number of days that rustup will backtrack when looking for a nightly
+  which suits the user's installation.
+
 - `RUSTUP_NO_BACKTRACE`
   Disables backtraces on non-panic errors even when `RUST_BACKTRACE` is set.
 

src/dist/dist.rs

@@ -605,6 +605,11 @@ fn update_from_dist_<'a>(
     let mut fetched = String::new();
     let mut first_err = None;
     let backtrack = toolchain.channel == "nightly" && toolchain.date.is_none();
+    let mut backtrack_attempts: i32 = env::var("RUSTUP_BACKTRACK_LIMIT")
+        .ok()
+        .unwrap_or_else(|| "60".to_string())
+        .parse()
+        .map_err(|e| format!("Bad number for RUSTUP_BACKTRACK_LIMIT: {}", e))?;
     loop {
         match try_update_from_dist_(
             download,
@@ -640,6 +645,17 @@ fn update_from_dist_<'a>(
                     break Err(e);
                 }
 
+                backtrack_attempts -= 1;
+                if backtrack_attempts < 1 {
+                    if let Some(e) = first_err {
+                        break Err(e);
+                    } else {
+                        break Err(Error::from(ErrorKind::MissingReleaseForToolchain(
+                            toolchain.channel,
+                        )));
+                    }
+                }
+
                 // The user asked to update their nightly, but the latest nightly does not have all
                 // the components that the user currently has installed. Let's try the previous
                 // nightlies in reverse chronological order until we find a nightly that does,