多线程下SBCL全局哈希表gethash返回NIL问题排查

问题现象 #

单线程模式下dictionary-compatible函数正常工作，但使用lparallel库的多线程新镜像时，对全局哈希表*lci-hash-table*的gethash调用始终返回nil。该哈希表仅用于读取，未做更新操作，按SBCL手册说明无需设置:synchronized或用sb-ext:defglobal定义。

核心代码 #

全局哈希表定义与核心函数 #

(defparameter *lci-hash-table* (make-hash-table :size 10000))  
  "Hash-table key = length|char|index concatenated together (eg, 5A0),
   hash-table values = list of matching symbols (eg, (ABATE ABBEY ...))."

(defun dictionary-compatible ($new-cross-str)  ;eg, "A     " of length 5
  "Tests if a string (with uppercase alphabetic and space characters)
   is compatible with the dictionary words (symbols) in a hash-table."
  (iter (with len = (length $new-cross-str))
        (for char in-sequence $new-cross-str)
        (for index from 0)
        (when (not (eql char #\Space))
          (collect (gethash (ut::intern-symbol len char index)  ;gethash始终返回nil
                            *lci-hash-table*) into dict-words))
        (finally (return (reduce #'intersection dict-words)))))

(defun intern-symbol (&rest args)
  "Interns a symbol created by concatenating args.
   Based on symb in Let Over Lambda."
  (flet ((mkstr (&rest args)
           (with-output-to-string (s)
             (dolist (a args) (princ a s)))))
    (values (intern (apply #'mkstr args)))))

哈希表填充代码 #

(defun create-lci-ht (dictionary-file)
  "Read dictionary words (symbols) from a file into strings
   and store in a lci hash table."
  (with-open-file (infile dictionary-file :direction :input :if-does-not-exist nil)
    (when (not (streamp infile)) (error "File does not exist!"))
    (with-open-file (stream dictionary-file)
      (let ((word-strings (uiop:read-file-lines stream)))
        (iter (for word-string in word-strings)
              (for word-length = (length word-string))
              (while word-string)
              (iter (for char in-sequence word-string)
                    (for index from 0)
                    (push (intern word-string)
                          (gethash (ut::intern-symbol word-length char index)
                                   *lci-hash-table*)))))))

问题根源 #

问题出在intern-symbol函数中的intern调用：该函数未指定固定的目标包，而是依赖于调用时的动态*package*变量。

• 填充哈希表时，若调用环境的当前包是UT，则intern会将生成的键符号存入UT包（如UT:5A0）。
• 多线程worker的运行环境中，当前包可能是默认的CL-USER，此时调用ut::intern-symbol时，内部的intern会将符号存入CL-USER包（如CL-USER:5A0）。

这两个符号属于不同包，即使名称相同，eql比较也会返回nil，导致gethash无法匹配到哈希表中的键，最终返回nil。

解决方案 #

修改intern-symbol函数，指定将符号intern到固定包（比如UT包），确保无论调用环境的当前包是什么，生成的键符号都一致：

(defun intern-symbol (&rest args)
  "Interns a symbol created by concatenating args into the UT package.
   Based on symb in Let Over Lambda."
  (flet ((mkstr (&rest args)
           (with-output-to-string (s)
             (dolist (a args) (princ a s)))))
    (values (intern (apply #'mkstr args) (find-package 'ut)))))

补充说明：也可以提前创建一个专门用于存储哈希表键的包，避免与其他包的符号冲突，原理是一致的。

内容的提问来源于stack exchange，提问作者davypough