Reset concurrency to the initial default by setting 0 or nil

`Vips.concurrency_set(0)` suggests it'll set concurrency back to the
default value, but it does nothing if concurrency is already set, which
is always true because Vips.init has already configured thread pools.

The result is that if you set concurrency to any value, setting it to
zero will leave that value in place.

This reads the default concurrency immediately after Vips.init so we
can make good on having `concurrency_set 0` reset to default. Also
allows setting `nil` to reset to default.

Note that setting concurrency does not reconfigure existing thread
pools. It affects thread pools spawned for future operations.

Author: jeremy

lib/vips.rb

@@ -626,6 +626,10 @@ def to_s
   attach_function :vips_leak_set, [:int], :void
   attach_function :vips_vector_set_enabled, [:int], :void
   attach_function :vips_concurrency_set, [:int], :void
+  attach_function :vips_concurrency_get, [], :int
+
+  # Track the original default concurrency so we can reset to it.
+  DEFAULT_CONCURRENCY = vips_concurrency_get
 
   # vips_foreign_get_suffixes was added in libvips 8.8
   begin
@@ -663,10 +667,23 @@ def self.cache_set_max_files size
     vips_cache_set_max_files size
   end
 
-  # Set the size of the libvips worker pool. This defaults to the number of
-  # hardware threads on your computer. Set to 1 to disable threading.
+  # Get the size of libvips worker pools. Defaults to the VIPS_CONCURRENCY env
+  # var or the number of hardware threads on your computer.
+  def self.concurrency
+    vips_concurrency_get
+  end
+
+  # Get the default size of libvips worker pools.
+  def self.concurrency_default
+    DEFAULT_CONCURRENCY
+  end
+
+  # Set the size of each libvips worker pool. Max 1024 threads. Set to 1 to
+  # disable threading. Set to 0 or nil to reset to default.
   def self.concurrency_set n
+    n = DEFAULT_CONCURRENCY if n.to_i == 0
     vips_concurrency_set n
+    concurrency
   end
 
   # Enable or disable SIMD and the run-time compiler. This can give a nice

spec/spec_helper.rb

@@ -1,3 +1,11 @@
+# Set default concurrency so we can check against it later. Must be set
+# before Vips.init sets concurrency to the default.
+DEFAULT_VIPS_CONCURRENCY = 5
+ENV["VIPS_CONCURRENCY"] = DEFAULT_VIPS_CONCURRENCY.to_s
+
+# Disable stderr output since we purposefully trigger warn-able behavior.
+ENV["VIPS_WARNING"] = "1"
+
 require "vips"
 
 require "tempfile"

spec/vips_spec.rb

@@ -2,8 +2,31 @@
 
 RSpec.describe Vips do
   describe "Vips" do
+    it "can get default concurrency" do
+      expect(Vips.concurrency_default).to eq DEFAULT_VIPS_CONCURRENCY
+    end
+
+    it "can get concurrency" do
+      expect(Vips.concurrency).to eq Vips.concurrency_default
+    end
+
     it "can set concurrency" do
-      Vips.concurrency_set 12
+      expect(Vips.concurrency_set(12)).to eq 12
+      expect(Vips.concurrency).to eq 12
+    end
+
+    it "clips concurrency to 1024" do
+      expect(Vips.concurrency_set(1025)).to eq 1024
+    end
+
+    it "can set concurrency to 0 to reset to default" do
+      Vips.concurrency_set(rand(100))
+      expect(Vips.concurrency_set(0)).to eq Vips.concurrency_default
+    end
+
+    it "can set concurrency to nil to reset to default" do
+      Vips.concurrency_set(rand(100))
+      expect(Vips.concurrency_set(nil)).to eq Vips.concurrency_default
     end
 
     it "can set SIMD" do