* After building, scan for actual file system references as

opposed to declared references.  This prunes the reference
  graph, thus allowing better garbage collection and more
  efficient derivate distribution.

src/Makefile.am

@@ -20,7 +20,7 @@ test_LDADD = libshared.a libnix.a -ldb_cxx-4 -lATerm
 noinst_LIBRARIES = libnix.a libshared.a
 
 libnix_a_SOURCES = util.cc hash.cc archive.cc md5.c \
- fstate.cc store.cc globals.cc db.cc
+ fstate.cc store.cc globals.cc db.cc references.cc
 
 libshared_a_SOURCES = shared.cc
 

src/archive.cc

@@ -87,7 +87,7 @@ static void dumpContents(const string & path, unsigned int size,
     writeInt(size, sink);
 
     int fd = open(path.c_str(), O_RDONLY);
-    if (fd == -1) throw SysError("opening file " + path);
+    if (fd == -1) throw SysError(format("opening file `%1%'") % path);
     
     unsigned char buf[65536];
 
@@ -112,7 +112,7 @@ static void dump(const string & path, DumpSink & sink)
 {
     struct stat st;
     if (lstat(path.c_str(), &st))
-        throw SysError("getting attributes of path " + path);
+        throw SysError(format("getting attributes of path `%1%'") % path);
 
     writeString("(", sink);
 

src/fstate.cc

@@ -11,6 +11,7 @@
 #include "globals.hh"
 #include "store.hh"
 #include "db.hh"
+#include "references.hh"
 
 
 /* A Unix environment is a mapping from strings to strings. */
@@ -279,12 +280,15 @@ static FState realise(FState fs, StringSet & paths)
         checkPlatform(platform);
         
         /* Realise inputs. */
+        Strings inPaths;
         ATermList ins2 = ATempty;
         while (!ATisEmpty(ins)) {
-            ins2 = ATinsert(ins2, realise(ATgetFirst(ins), paths));
+            FState in = realise(ATgetFirst(ins), paths);
+            inPaths.push_back(fstatePath(in));
+            ins2 = ATinsert(ins2, in);
             ins = ATgetNext(ins);
         }
-        ins2 = ATreverse(ins2);
+        ins = ATreverse(ins2);
 
         /* Build the environment. */
         Environment env;
@@ -323,9 +327,34 @@ static FState realise(FState fs, StringSet & paths)
            values.cc. */
         registerPath(outPath, outHash);
 
+        /* Filter out inputs that are not referenced in the output. */
+        for (Strings::iterator i = inPaths.begin();
+             i != inPaths.end(); i++)
+            debug(format("in: %1%") % *i);
+
+        Strings outPaths = filterReferences(outPath, inPaths);
+
+        for (Strings::iterator i = outPaths.begin();
+             i != outPaths.end(); i++)
+            debug(format("out: %1%") % *i);
+
+        ins2 = ATempty;
+        while (!ATisEmpty(ins)) {
+            FState in = ATgetFirst(ins);
+            string path = fstatePath(in);
+            for (Strings::iterator i = outPaths.begin();
+                 i != outPaths.end(); i++)
+                if (path.find(*i) != string::npos) {
+                    debug(format("out2: %1%") % path);
+                    ins2 = ATinsert(ins2, in);
+                }
+            ins = ATgetNext(ins);
+        }
+        ins = ATreverse(ins2);
+        
         /* Register the normal form of fs. */
         FState nf = ATmake("Path(<str>, Hash(<str>), <term>)",
-            outPath.c_str(), ((string) outHash).c_str(), ins2);
+            outPath.c_str(), ((string) outHash).c_str(), ins);
         nf = storeSuccessor(fs, nf, paths);
 
         return nf;

src/fstate.hh

@@ -1,5 +1,5 @@
-#ifndef __EVAL_H
+#ifndef __FSTATE_H
-#define __EVAL_H
+#define __FSTATE_H
 
 #include <set>
 
@@ -97,4 +97,4 @@ Hash writeTerm(ATerm t, const string & suffix, string * p = 0);
 void registerSuccessor(const Hash & fsHash, const Hash & scHash);
 
 
-#endif /* !__EVAL_H */
+#endif /* !__FSTATE_H */

src/references.cc

@@ -0,0 +1,98 @@
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <dirent.h>
+#include <fcntl.h>
+
+#include "references.hh"
+#include "hash.hh"
+
+
+static void search(const string & s,
+    Strings & refs, Strings & seen)
+{
+    for (Strings::iterator i = refs.begin();
+         i != refs.end(); )
+    {
+        if (s.find(*i) == string::npos)
+            i++;
+        else {
+            debug(format("found reference to `%1%'") % *i);
+            seen.push_back(*i);
+            i = refs.erase(i);
+        }
+    }
+}
+
+
+void checkPath(const string & path,
+    Strings & refs, Strings & seen)
+{
+    struct stat st;
+    if (lstat(path.c_str(), &st))
+        throw SysError(format("getting attributes of path `%1%'") % path);
+
+    if (S_ISDIR(st.st_mode)) {
+        DIR * dir = opendir(path.c_str());
+
+        struct dirent * dirent;
+        while (errno = 0, dirent = readdir(dir)) {
+            string name = dirent->d_name;
+            if (name == "." || name == "..") continue;
+            search(name, refs, seen);
+            checkPath(path + "/" + name, refs, seen);
+        }
+
+        closedir(dir); /* !!! close on exception */
+    }
+
+    else if (S_ISREG(st.st_mode)) {
+        
+        debug(format("checking `%1%'") % path);
+
+        int fd = open(path.c_str(), O_RDONLY);
+        if (fd == -1) throw SysError(format("opening file `%1%'") % path);
+
+        char * buf = new char[st.st_size];
+
+        if (read(fd, buf, st.st_size) != st.st_size)
+            throw SysError(format("reading file %1%") % path);
+
+        search(string(buf, st.st_size), refs, seen);
+        
+        delete buf; /* !!! autodelete */
+
+        close(fd); /* !!! close on exception */
+    }
+    
+    else if (S_ISLNK(st.st_mode)) {
+        char buf[st.st_size];
+        if (readlink(path.c_str(), buf, st.st_size) != st.st_size)
+            throw SysError(format("reading symbolic link `%1%'") % path);
+        search(string(buf, st.st_size), refs, seen);
+    }
+    
+    else throw Error(format("unknown file type: %1%") % path);
+}
+
+
+Strings filterReferences(const string & path, const Strings & _refs)
+{
+    Strings refs;
+    Strings seen;
+
+    /* For efficiency (and a higher hit rate), just search for the
+       hash part of the file name.  (This assumes that all references
+       have the form `HASH-bla'). */
+    for (Strings::const_iterator i = _refs.begin();
+         i != _refs.end(); i++)
+    {
+        string s = string(baseNameOf(*i), 0, 32);
+        parseHash(s);
+        refs.push_back(s);
+    }
+
+    checkPath(path, refs, seen);
+
+    return seen;
+}

src/references.hh

@@ -0,0 +1,10 @@
+#ifndef __VALUES_H
+#define __VALUES_H
+
+#include "util.hh"
+
+
+Strings filterReferences(const string & path, const Strings & refs);
+
+
+#endif /* !__VALUES_H */

src/store.hh

@@ -1,5 +1,5 @@
-#ifndef __VALUES_H
+#ifndef __STORE_H
-#define __VALUES_H
+#define __STORE_H
 
 #include <string>
 
@@ -37,4 +37,4 @@ void addToStore(string srcPath, string & dstPath, Hash & hash,
 void deleteFromStore(const string & path);
 
 
-#endif /* !__VALUES_H */
+#endif /* !__STORE_H */