Re-enable compatibility with readonly CARGO_HOME

Previously Cargo would attempt to work as much as possible with a
previously filled out CARGO_HOME, even if it was mounted as read-only.
In #6880 this was regressed as a few global locks and files were always
attempted to be opened in writable mode.

This commit fixes these issues by correcting two locations:

* First the global package cache lock has error handling to allow
  acquiring the lock in read-only mode inaddition to read/write mode. If
  the read/write mode failed due to an error that looks like a readonly
  filesystem then we assume everything in the package cache is readonly
  and we switch to just acquiring any lock, this time a shared readonly
  one. We in theory aren't actually doing any synchronization at that
  point since it's all readonly anyway.

* Next when unpacking package we're careful to issue a `stat` call
  before opening a file in writable mode. This way our preexisting guard
  to return early if a package is unpacked will succeed before we open
  anything in writable mode.

Closes #6928

src/cargo/sources/registry/mod.rs

@@ -449,15 +449,16 @@ impl<'cfg> RegistrySource<'cfg> {
         let path = dst.join(PACKAGE_SOURCE_LOCK);
         let path = self.config.assert_package_cache_locked(&path);
         let unpack_dir = path.parent().unwrap();
+        if let Ok(meta) = path.metadata() {
+            if meta.len() > 0 {
+                return Ok(unpack_dir.to_path_buf());
+            }
+        }
         let mut ok = OpenOptions::new()
             .create(true)
             .read(true)
             .write(true)
             .open(&path)?;
-        let meta = ok.metadata()?;
-        if meta.len() > 0 {
-            return Ok(unpack_dir.to_path_buf());
-        }
 
         let gz = GzDecoder::new(tarball);
         let mut tar = Archive::new(gz);

src/cargo/util/config.rs

@@ -6,7 +6,7 @@ use std::env;
 use std::fmt;
 use std::fs::{self, File};
 use std::io::prelude::*;
-use std::io::SeekFrom;
+use std::io::{self, SeekFrom};
 use std::mem;
 use std::path::{Path, PathBuf};
 use std::str::FromStr;
@@ -860,21 +860,71 @@ impl Config {
         return ret;
     }
 
+    /// Acquires an exclusive lock on the global "package cache"
+    ///
+    /// This lock is global per-process and can be acquired recursively. An RAII
+    /// structure is returned to release the lock, and if this process
+    /// abnormally terminates the lock is also released.
     pub fn acquire_package_cache_lock<'a>(&'a self) -> CargoResult<PackageCacheLock<'a>> {
         let mut slot = self.package_cache_lock.borrow_mut();
         match *slot {
+            // We've already acquired the lock in this process, so simply bump
+            // the count and continue.
             Some((_, ref mut cnt)) => {
                 *cnt += 1;
             }
             None => {
-                let lock = self
+                let path = ".package-cache";
-                    .home_path
+                let desc = "package cache lock";
-                    .open_rw(".package-cache", self, "package cache lock")
+
-                    .chain_err(|| "failed to acquire package cache lock")?;
+                // First, attempt to open an exclusive lock which is in general
-                *slot = Some((lock, 1));
+                // the purpose of this lock!
+                //
+                // If that fails because of a readonly filesystem, though, then
+                // we don't want to fail because it's a readonly filesystem. In
+                // some situations Cargo is prepared to have a readonly
+                // filesystem yet still work since it's all been pre-downloaded
+                // and/or pre-unpacked. In these situations we want to keep
+                // Cargo running if possible, so if it's a readonly filesystem
+                // switch to a shared lock which should hopefully succeed so we
+                // can continue.
+                //
+                // Note that the package cache lock protects files in the same
+                // directory, so if it's a readonly filesystem we assume that
+                // the entire package cache is readonly, so we're just acquiring
+                // something to prove it works, we're not actually doing any
+                // synchronization at that point.
+                match self.home_path.open_rw(path, self, desc) {
+                    Ok(lock) => *slot = Some((lock, 1)),
+                    Err(e) => {
+                        if maybe_readonly(&e) {
+                            if let Ok(lock) = self.home_path.open_ro(path, self, desc) {
+                                *slot = Some((lock, 1));
+                                return Ok(PackageCacheLock(self));
+                            }
+                        }
+
+                        Err(e).chain_err(|| "failed to acquire package cache lock")?;
+                    }
+                }
             }
         }
-        Ok(PackageCacheLock(self))
+        return Ok(PackageCacheLock(self));
+
+        fn maybe_readonly(err: &failure::Error) -> bool {
+            err.iter_chain().any(|err| {
+                if let Some(io) = err.downcast_ref::<io::Error>() {
+                    if io.kind() == io::ErrorKind::PermissionDenied {
+                        return true;
+                    }
+
+                    #[cfg(unix)]
+                    return io.raw_os_error() == Some(libc::EROFS);
+                }
+
+                false
+            })
+        }
     }
 
     pub fn release_package_cache_lock(&self) {}

tests/testsuite/registry.rs

@@ -1,5 +1,6 @@
 use std::fs::{self, File};
 use std::io::prelude::*;
+use std::path::Path;
 
 use crate::support::cargo_process;
 use crate::support::git;
@@ -1979,3 +1980,48 @@ fn ignore_invalid_json_lines() {
 
     p.cargo("build").run();
 }
+
+#[test]
+fn readonly_registry_still_works() {
+    Package::new("foo", "0.1.0").publish();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "a"
+                version = "0.5.0"
+                authors = []
+
+                [dependencies]
+                foo = '0.1.0'
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("generate-lockfile").run();
+    p.cargo("fetch --locked").run();
+    chmod_readonly(&paths::home());
+    p.cargo("build").run();
+
+    fn chmod_readonly(path: &Path) {
+        for entry in t!(path.read_dir()) {
+            let entry = t!(entry);
+            let path = entry.path();
+            if t!(entry.file_type()).is_dir() {
+                chmod_readonly(&path);
+            } else {
+                set_readonly(&path);
+            }
+        }
+        set_readonly(path);
+    }
+
+    fn set_readonly(path: &Path) {
+        let mut perms = t!(path.metadata()).permissions();
+        perms.set_readonly(true);
+        t!(fs::set_permissions(path, perms));
+    }
+}

tests/testsuite/support/paths.rs

@@ -158,10 +158,18 @@ where
 {
     match f(path) {
         Ok(()) => {}
-        Err(ref e) if cfg!(windows) && e.kind() == ErrorKind::PermissionDenied => {
+        Err(ref e) if e.kind() == ErrorKind::PermissionDenied => {
             let mut p = t!(path.metadata()).permissions();
             p.set_readonly(false);
             t!(fs::set_permissions(path, p));
+
+            // Unix also requires the parent to not be readonly for example when
+            // removing files
+            let parent = path.parent().unwrap();
+            let mut p = t!(parent.metadata()).permissions();
+            p.set_readonly(false);
+            t!(fs::set_permissions(parent, p));
+
             f(path).unwrap_or_else(|e| {
                 panic!("failed to {} {}: {}", desc, path.display(), e);
             })