Open README with write permissions

Evidently, some file systems require write permissions in order to lock
a file or locking will fail. Open the README file, which we use for
locking purposes, with write permissions in an attempt to make locking
work on all file systems that the crate may be built on.

Closes: libbpf#116

Signed-off-by: Daniel Müller <[email protected]>

Author: d-e-s-o

build.rs

@@ -4,6 +4,8 @@ use std::env;
 use std::ffi;
 use std::fs;
 use std::fs::read_dir;
+use std::fs::File;
+use std::io;
 use std::path;
 use std::path::Path;
 use std::process;
@@ -208,12 +210,31 @@ fn main() {
     }
 }
 
+fn open_lockable(path: &Path) -> io::Result<File> {
+    let result = File::options()
+        .read(true)
+        // Open with write permissions because flock(2) may require them
+        // on some platforms.
+        .write(true)
+        .open(path);
+    match result {
+        Ok(file) => Ok(file),
+        Err(err) if err.kind() == io::ErrorKind::PermissionDenied => {
+            // On a read-only file system we may not be able to open
+            // with write permissions. So just open for reading and hope
+            // for the best.
+            File::open(path)
+        },
+        e @ Err(..) => e,
+    }
+}
+
 fn make_zlib(compiler: &cc::Tool, src_dir: &path::Path, out_dir: &path::Path) {
     let src_dir = src_dir.join("zlib");
     // lock README such that if two crates are trying to compile
     // this at the same time (eg libbpf-rs libbpf-cargo)
     // they wont trample each other
-    let file = std::fs::File::open(src_dir.join("README")).unwrap();
+    let file = open_lockable(&src_dir.join("README")).unwrap();
     let _lock = fcntl::Flock::lock(file, fcntl::FlockArg::LockExclusive).unwrap();
 
     let status = process::Command::new("./configure")
@@ -254,7 +275,7 @@ fn make_elfutils(compiler: &cc::Tool, src_dir: &path::Path, out_dir: &path::Path
     // lock README such that if two crates are trying to compile
     // this at the same time (eg libbpf-rs libbpf-cargo)
     // they wont trample each other
-    let file = std::fs::File::open(src_dir.join("elfutils/README")).unwrap();
+    let file = open_lockable(&src_dir.join("elfutils/README")).unwrap();
     let _lock = fcntl::Flock::lock(file, fcntl::FlockArg::LockExclusive).unwrap();
 
     let flags = compiler