From c8626bec8eb8e645d7d8b4add57c506dc7c6dd4e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Johannes=20M=C3=BCller?= <straightshoota@gmail.com>
Date: Sat, 5 Nov 2022 21:48:08 +0100
Subject: [PATCH 1/2] Implement `flock_*` fiber-aware, without blocking the
 thread

---
 spec/std/file_spec.cr           | 38 +++++++++++++++++++++++++++++++--
 src/crystal/system/unix/file.cr | 25 +++++++++++++++++-----
 src/io/file_descriptor.cr       |  1 -
 3 files changed, 56 insertions(+), 8 deletions(-)

diff --git a/spec/std/file_spec.cr b/spec/std/file_spec.cr
index 54993835500d..9c577f231b7e 100644
--- a/spec/std/file_spec.cr
+++ b/spec/std/file_spec.cr
@@ -940,10 +940,10 @@ describe "File" do
       File.open(datapath("test_file.txt")) do |file1|
         File.open(datapath("test_file.txt")) do |file2|
           file1.flock_exclusive do
-            # BUG: check for EWOULDBLOCK when exception filters are implemented
-            expect_raises(IO::Error, "Error applying or removing file lock") do
+            exc = expect_raises(IO::Error, "Error applying file lock: file is already locked") do
               file2.flock_exclusive(blocking: false) { }
             end
+            exc.os_error.should eq Errno::EWOULDBLOCK
           end
         end
       end
@@ -958,6 +958,40 @@ describe "File" do
         end
       end
     end
+
+    it "#flock_shared non-blocking" do
+      File.open(datapath("test_file.txt")) do |file1|
+        File.open(datapath("test_file.txt")) do |file2|
+          done = Channel(Nil).new
+          file1.flock_exclusive
+
+          spawn do
+            file1.flock_unlock
+            done.send nil
+          end
+
+          file2.flock_shared
+          done.receive
+        end
+      end
+    end
+
+    it "#flock_exclusive non-blocking" do
+      File.open(datapath("test_file.txt")) do |file1|
+        File.open(datapath("test_file.txt")) do |file2|
+          done = Channel(Nil).new
+          file1.flock_exclusive
+
+          spawn do
+            file1.flock_unlock
+            done.send nil
+          end
+
+          file2.flock_exclusive
+          done.receive
+        end
+      end
+    end
   end
 
   it "reads at offset" do
diff --git a/src/crystal/system/unix/file.cr b/src/crystal/system/unix/file.cr
index a73a190f45a5..87ff554a3b99 100644
--- a/src/crystal/system/unix/file.cr
+++ b/src/crystal/system/unix/file.cr
@@ -252,14 +252,29 @@ module Crystal::System::File
     flock LibC::FlockOp::UN
   end
 
-  private def flock(op : LibC::FlockOp, blocking : Bool = true)
-    op |= LibC::FlockOp::NB unless blocking
+  private def flock(op : LibC::FlockOp, retry : Bool) : Nil
+    op |= LibC::FlockOp::NB
 
-    if LibC.flock(fd, op) != 0
-      raise IO::Error.from_errno("Error applying or removing file lock")
+    if retry
+      until flock(op)
+        ::Fiber.yield
+      end
+    else
+      flock(op) || raise IO::Error.from_errno("Error applying file lock: file is already locked")
     end
+  end
 
-    nil
+  private def flock(op) : Bool
+    if 0 == LibC.flock(fd, op)
+      true
+    else
+      errno = Errno.value
+      if errno.in?(Errno::EAGAIN, Errno::EWOULDBLOCK)
+        false
+      else
+        raise IO::Error.from_os_error("Error applying or removing file lock", errno)
+      end
+    end
   end
 
   private def system_fsync(flush_metadata = true) : Nil
diff --git a/src/io/file_descriptor.cr b/src/io/file_descriptor.cr
index 6da5b859f77b..ecbcc6cebf88 100644
--- a/src/io/file_descriptor.cr
+++ b/src/io/file_descriptor.cr
@@ -175,7 +175,6 @@ class IO::FileDescriptor < IO
   end
 
   # TODO: use fcntl/lockf instead of flock (which doesn't lock over NFS)
-  # TODO: always use non-blocking locks, yield fiber until resource becomes available
 
   def flock_shared(blocking = true)
     flock_shared blocking

From 5b34a25d14b66d03eeabfc98491263c00313d786 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Johannes=20M=C3=BCller?= <straightshoota@gmail.com>
Date: Fri, 18 Nov 2022 14:28:06 +0100
Subject: [PATCH 2/2] Reword tests

---
 spec/std/file_spec.cr | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/spec/std/file_spec.cr b/spec/std/file_spec.cr
index 9c577f231b7e..74dc5007c415 100644
--- a/spec/std/file_spec.cr
+++ b/spec/std/file_spec.cr
@@ -959,7 +959,7 @@ describe "File" do
       end
     end
 
-    it "#flock_shared non-blocking" do
+    pending_win32 "#flock_shared soft blocking fiber" do
       File.open(datapath("test_file.txt")) do |file1|
         File.open(datapath("test_file.txt")) do |file2|
           done = Channel(Nil).new
@@ -976,7 +976,7 @@ describe "File" do
       end
     end
 
-    it "#flock_exclusive non-blocking" do
+    pending_win32 "#flock_exclusive soft blocking fiber" do
       File.open(datapath("test_file.txt")) do |file1|
         File.open(datapath("test_file.txt")) do |file2|
           done = Channel(Nil).new