fix(loggly): fix the error caused by the missing /bin/hostname (#13788)

* fix(loggly): fix the error caused by the missing /bin/hostname

1. Refactor the PDK's get_hostname to use only the gethostname system call to retrieve the hostname.
2. Loggly uses the PDK's get_hostname to retrieve the hostname.

FTI-6046

changelog/unreleased/kong/fix-loggly-hostname-notfound.yml

@@ -0,0 +1,2 @@
+message: "**Loggly**: Fixed an issue where `/bin/hostname` missing caused an error warning on startup."
+type: bugfix

kong/pdk/node.lua

@@ -57,7 +57,9 @@ end
 
 
 local function new(self)
-  local _NODE = {}
+  local _NODE = {
+    hostname = nil,
+  }
 
 
   ---
@@ -247,20 +249,23 @@ local function new(self)
   -- @usage
   -- local hostname = kong.node.get_hostname()
   function _NODE.get_hostname()
-    local SIZE = 253 -- max number of chars for a hostname
+    if not _NODE.hostname then
+      local SIZE = 253 -- max number of chars for a hostname
 
-    local buf = ffi_new("unsigned char[?]", SIZE)
-    local res = C.gethostname(buf, SIZE)
+      local buf = ffi_new("unsigned char[?]", SIZE)
+      local res = C.gethostname(buf, SIZE)
 
-    if res == 0 then
-      local hostname = ffi_str(buf, SIZE)
-      return gsub(hostname, "%z+$", "")
+      if res ~= 0 then
+        -- Return an empty string "" instead of nil and error message,
+        -- because strerror is not thread-safe and the behavior of strerror_r
+        -- is inconsistent across different systems.
+        return ""
+      end
+
+      _NODE.hostname = gsub(ffi_str(buf, SIZE), "%z+$", "")
     end
 
-    local f = io.popen("/bin/hostname")
-    local hostname = f:read("*a") or ""
-    f:close()
-    return gsub(hostname, "\n$", "")
+    return _NODE.hostname
   end
 
 

kong/plugins/loggly/handler.lua

@@ -1,6 +1,7 @@
 local cjson = require "cjson"
 local sandbox = require "kong.tools.sandbox".sandbox
 local kong_meta = require "kong.meta"
+local get_host_name = kong.node.get_hostname
 
 
 local kong = kong
@@ -16,15 +17,6 @@ local insert = table.insert
 local sandbox_opts = { env = { kong = kong, ngx = ngx } }
 
 
-local function get_host_name()
-  local f = io.popen("/bin/hostname")
-  local hostname = f:read("*a") or ""
-  f:close()
-  hostname = string.gsub(hostname, "\n$", "")
-  return hostname
-end
-
-
 local HOSTNAME = get_host_name()
 local SENDER_NAME = "kong"
 local LOG_LEVELS = {