feat: configurable document root for UFS-backed HTTP serving (#24)

Add httpd.docroot Lua config option (e.g. docroot="/www") that
prefixes all UFS file paths in HTTP requests. This allows mounting
a dedicated UFS disk on a subdirectory via UFSD parmlib and pointing
HTTPD at it.

- httpd.h: add docroot[128] field to HTTPD struct
- httpconf.c: read httpd.docroot from Lua config, default empty
- httpopen.c: prepend docroot to UFS paths in http_open (skipped
  for /DD: paths which bypass UFS)

Example config:
  httpd.docroot = "/www"

With UFSD parmlib:
  MOUNT DSN(HTTPD.WEBROOT) PATH(/www) MODE(RO)

URL /index.html resolves to UFS path /www/index.html.

Author: mgrossmann

include/httpd.h

@@ -150,7 +150,8 @@ struct httpd {
 #define HTTPD_CGICTX_MVSMF  0       /* ... MVSMF CGI context index  */
 #define HTTPD_CGICTX_MIN    0       /* ... minimum number of cgictx */
 #define HTTPD_CGICTX_MAX    255     /* ... maximum number of cgictx */
-};                                  /* A0 (160 bytes)               */
+    char        docroot[128];       /* A0 UFS document root prefix  */
+};                                  /* 120                          */
 
 /* Telemetry */
 struct httpt {

src/httpconf.c

@@ -547,6 +547,24 @@ process_httpd_ufs(lua_State *L, HTTPD *httpd)
 
     if (crt) crt->crtufs = ufs;
 
+    /* Read document root prefix (optional) */
+    lua_getfield(L, -1, "docroot");
+    {
+        const char *dr = lua_tostring(L, -1);
+        if (dr && dr[0] == '/') {
+            int drlen = strlen(dr);
+            /* strip trailing slash */
+            if (drlen > 1 && dr[drlen - 1] == '/')
+                drlen--;
+            if (drlen >= (int)sizeof(httpd->docroot))
+                drlen = (int)sizeof(httpd->docroot) - 1;
+            memcpy(httpd->docroot, dr, drlen);
+            httpd->docroot[drlen] = '\0';
+            wtof("HTTPD047I Document root: %s", httpd->docroot);
+        }
+    }
+    lua_pop(L, 1);
+
 quit:
 	return rc;
 }
@@ -1094,6 +1112,9 @@ setHttpdDefaults(lua_State *L)
 
 	lua_pushstring(L, "1");
 	lua_setfield(L,-2,"ufs");		// table.ufs="1"
+
+	lua_pushstring(L, "");
+	lua_setfield(L,-2,"docroot");	// table.docroot=""
 
 	lua_pushstring(L, "HTTPD.CGILUA");
 	lua_setfield(L,-2,"cgilua_dataset");	// table.cgilua_dataset="HTTPD.CGILUA"

src/httpopen.c

@@ -98,11 +98,16 @@ http_open(HTTPC *httpc, const UCHAR *path, const HTTPM *mime)
 
 
     if (httpc->ufs) {
-        httpc->ufp = ufs_fopen(httpc->ufs, buf, mode);
-		// wtof("%s: ufs_fopen(%p, \"%s\", \"%s\") httpc->ufp=%p", 
-		// 	__func__, httpc->ufs, buf, mode, httpc->ufp);
+        UCHAR ufspath[256];
+        const char *dr = httpc->httpd->docroot;
+        if (dr[0] && http_cmpn(buf, "/DD:", 4) != 0) {
+            /* prepend document root to UFS path */
+            snprintf((char *)ufspath, sizeof(ufspath), "%s%s", dr, buf);
+            httpc->ufp = ufs_fopen(httpc->ufs, ufspath, mode);
+        } else {
+            httpc->ufp = ufs_fopen(httpc->ufs, buf, mode);
+        }
         if (httpc->ufp) {
-            /* wtof("%s opened \"%s\" UFS", __func__, buf); */
             goto quit;  /* success, return NULL to caller */
         }
     }