DEV: add tmp_dir_available_bytes metric

The Discourse application and plugins utilise the `/tmp` directory for
local storage during request and job processing. If the directory has no
space remaining, attempting to create files will fail.

Exposing the amount of space available in the directory is useful in
environments where this directory is limited in size.

See t/111786.

Author: fitzy101

lib/collector.rb

@@ -185,6 +185,11 @@ def ensure_global_metrics
         "The highest last_value from the pg_sequences table",
       )
 
+      global_metrics << Gauge.new(
+        "tmp_dir_available_bytes",
+        "Available space in /tmp directory (bytes)",
+      )
+
       @global_metrics = global_metrics
     end
 

lib/internal_metric/global.rb

@@ -27,7 +27,8 @@ class Global < Base
               :missing_s3_uploads,
               :version_info,
               :readonly_sites,
-              :postgres_highest_sequence
+              :postgres_highest_sequence,
+              :tmp_dir_available_bytes
 
     def initialize
       @active_app_reqs = 0
@@ -158,6 +159,8 @@ def collect
       @readonly_sites = collect_readonly_sites
 
       @postgres_highest_sequence = calc_postgres_highest_sequence
+
+      @tmp_dir_available_bytes = collect_dir_stats("/tmp")
     end
 
     # For testing purposes
@@ -180,6 +183,21 @@ def collect_readonly_sites
       result
     end
 
+    def collect_dir_stats(dir)
+      stdout, status = Open3.capture2("df", "-B1", dir)
+      return nil if !status.success?
+
+      begin
+        dirstat = stdout.lines[-1].split()
+        dirstat[3].to_i
+      rescue Exception => e
+        Discourse.warn_exception(
+          e,
+          message: "Failed to read disk usage for tmp_dir_available_bytes metric",
+        )
+      end
+    end
+
     def primary_site_readonly?
       return 1 if !defined?(Discourse::PG_READONLY_MODE_KEY)
       Discourse.redis.without_namespace.get("default:#{Discourse::PG_READONLY_MODE_KEY}") ? 1 : 0

spec/lib/internal_metric/global_spec.rb

@@ -14,6 +14,7 @@
     expect(metric.postgres_replica_available).to eq(nil)
     expect(metric.redis_primary_available).to eq({ { type: "main" } => 1 })
     expect(metric.redis_replica_available).to eq({ { type: "main" } => 0 })
+    expect(metric.tmp_dir_available_bytes).to be > 0
   end
 
   it "collects the version_info metric" do