fix: per-toolchain installation lock (#144)

```
$ elan install nightly
info: waiting for previous installation request to finish (/home/sebastian/.elan/toolchains/leanprover--lean4-nightly---nightly-2024-11-23.lock, held by PID 2508083
)

leanprover/lean4-nightly:nightly-2024-11-23 installed
```

src/elan-dist/Cargo.toml

@@ -26,6 +26,7 @@ json = "0.12.4"
 zip = "0.6"
 filetime = "0.2.14"
 time = "0.3"
+fslock = "0.2.1"
 
 [target."cfg(windows)".dependencies]
 winapi = { version = "0.3.9", features = ["handleapi", "sysinfoapi", "tlhelp32", "winnt"] }
@@ -36,4 +37,3 @@ libc = "0.2.88"
 
 [lib]
 name = "elan_dist"
-

src/elan-dist/src/lib.rs

@@ -1,5 +1,6 @@
 #![recursion_limit = "1024"]
 
+extern crate fslock;
 extern crate elan_utils;
 extern crate flate2;
 extern crate itertools;

src/elan-dist/src/manifestation.rs

@@ -1,9 +1,12 @@
 //! Manifest a particular Lean version by installing it from a distribution server.
 
+use std::{thread::sleep, time::Duration};
+
 use component::{TarGzPackage, TarZstdPackage, ZipPackage};
 use download::DownloadCfg;
-use elan_utils::utils;
+use elan_utils::{raw::read_file, utils};
 use errors::*;
+use fslock::LockFile;
 use notifications::*;
 use prefix::InstallPrefix;
 use temp;
@@ -25,11 +28,28 @@ impl Manifestation {
         temp_cfg: &temp::Cfg,
         notify_handler: &dyn Fn(Notification),
     ) -> Result<()> {
+        let prefix = self.prefix.path();
+        utils::ensure_dir_exists("toolchains", prefix.parent().unwrap(), &|n| {
+            (notify_handler)(n.into())
+        })?;
+
+        let lockfile_path = prefix.with_extension("lock");
+        let mut lockfile = LockFile::open(&lockfile_path)?;
+        if !lockfile.try_lock_with_pid()? {
+            notify_handler(Notification::WaitingForFileLock(&lockfile_path, read_file(&lockfile_path)?.trim()));
+            while !lockfile.try_lock_with_pid()? {
+                sleep(Duration::from_secs(1));
+            }
+        }
         let dlcfg = DownloadCfg {
             temp_cfg: temp_cfg,
             notify_handler: notify_handler,
         };
 
+        if utils::is_directory(prefix) {
+            return Ok(())
+        }
+
         // find correct download on HTML page (AAAAH)
         use regex::Regex;
         use std::fs;
@@ -70,17 +90,11 @@ impl Manifestation {
 
         let installer_file = dlcfg.download_and_check(&url)?;
 
-        let prefix = self.prefix.path();
-
         notify_handler(Notification::InstallingComponent(&prefix.to_string_lossy()));
 
         // unpack into temporary place, then move atomically to guard against aborts during unpacking
         let unpack_dir = prefix.with_extension("tmp");
 
-        if utils::is_directory(prefix) {
-            return Err(format!("'{}' is already installed", prefix.display()).into());
-        }
-
         if utils::is_directory(&unpack_dir) {
             utils::remove_dir("temp toolchain directory", &unpack_dir, &|n| {
                 (notify_handler)(n.into())
@@ -103,6 +117,7 @@ impl Manifestation {
         }
 
         utils::rename_dir("temp toolchain directory", &unpack_dir, prefix)?;
+        let _ = std::fs::remove_file(&lockfile_path);
 
         Ok(())
     }

src/elan-dist/src/notifications.rs

@@ -31,6 +31,7 @@ pub enum Notification<'a> {
     DownloadingLegacyManifest,
     ManifestChecksumFailedHack,
     NewVersionAvailable(String),
+    WaitingForFileLock(&'a Path, &'a str),
 }
 
 impl<'a> From<elan_utils::Notification<'a>> for Notification<'a> {
@@ -65,6 +66,7 @@ impl<'a> Notification<'a> {
             | RollingBack
             | DownloadingManifest(_)
             | NewVersionAvailable(_)
+            | WaitingForFileLock(_, _)
             | DownloadedManifest(_, _) => NotificationLevel::Info,
             CantReadUpdateHash(_)
             | ExtensionNotInstalled(_)
@@ -125,6 +127,9 @@ impl<'a> Display for Notification<'a> {
                     "Version {version} of elan is available! Use `elan self update` to update."
                 )
             }
+            WaitingForFileLock(path, pid) => {
+                write!(f, "waiting for previous installation request to finish ({}, held by PID {})", path.display(), pid)
+            }
         }
     }
 }