pcre: remove nthreads constant dependency (#44542)

This appears to be the only remaining place we use the nthreads variable
to pre-initialize a shared array. Let us fix that code pattern now,
since we know globals are already acquire/release.

Author: vtjnash

base/Base.jl

@@ -507,6 +507,10 @@ function __init__()
     nothing
 end
 
+# enable threads support
+@eval PCRE PCRE_COMPILE_LOCK = Threads.SpinLock()
+
 end
 
+
 end # baremodule Base

base/client.jl

@@ -514,10 +514,6 @@ MainInclude.include
 function _start()
     empty!(ARGS)
     append!(ARGS, Core.ARGS)
-    if ccall(:jl_generating_output, Cint, ()) != 0 && JLOptions().incremental == 0
-        # clear old invalid pointers
-        PCRE.__init__()
-    end
     try
         exec_options(JLOptions())
     catch

base/pcre.jl

@@ -24,28 +24,47 @@ function create_match_context()
     return ctx
 end
 
-const THREAD_MATCH_CONTEXTS = Ptr{Cvoid}[C_NULL]
+THREAD_MATCH_CONTEXTS::Vector{Ptr{Cvoid}} = [C_NULL]
 
 PCRE_COMPILE_LOCK = nothing
 
-_tid() = Int(ccall(:jl_threadid, Int16, ())+1)
+_tid() = Int(ccall(:jl_threadid, Int16, ())) + 1
 _nth() = Int(unsafe_load(cglobal(:jl_n_threads, Cint)))
 
 function get_local_match_context()
+    global THREAD_MATCH_CONTEXTS
     tid = _tid()
-    ctx = @inbounds THREAD_MATCH_CONTEXTS[tid]
+    ctxs = THREAD_MATCH_CONTEXTS
+    if length(ctxs) < tid
+        # slow path to allocate it
+        l = PCRE_COMPILE_LOCK::Threads.SpinLock
+        lock(l)
+        try
+            THREAD_MATCH_CONTEXTS = ctxs = copyto!(fill(C_NULL, _nth()), THREAD_MATCH_CONTEXTS)
+        finally
+            unlock(l)
+        end
+    end
+    ctx = @inbounds ctxs[tid]
     if ctx == C_NULL
-        @inbounds THREAD_MATCH_CONTEXTS[tid] = ctx = create_match_context()
+        # slow path to allocate it
+        ctx = create_match_context()
+        l = PCRE_COMPILE_LOCK
+        if l === nothing
+            THREAD_MATCH_CONTEXTS[tid] = ctx
+        else
+            l = l::Threads.SpinLock
+            lock(l)
+            try
+                THREAD_MATCH_CONTEXTS[tid] = ctx
+            finally
+                unlock(l)
+            end
+        end
     end
     return ctx
 end
 
-function __init__()
-    resize!(THREAD_MATCH_CONTEXTS, _nth())
-    fill!(THREAD_MATCH_CONTEXTS, C_NULL)
-    global PCRE_COMPILE_LOCK = Threads.SpinLock()
-end
-
 # supported options for different use cases
 
 # arguments to pcre2_compile