tame: Introduce __pkguniq and preproc:pkg-generate-id to replace generate-id

This modifies the XSLT-based compiler to generate ids that are expected to
be unique across packages.  No such guarantee exists today; `generate-id()`
relies on the position of the node within a tree, which could easily be the
same across multiple compiler invocations for separate packages.

This situation seldom occurs, but has happened with increased frequency
lately in a system with >1000 packages.  It is more likely to occur in
packages that are very similar to one-another or where the beginning of the
package is similar (such as packages used as configuration for taxes for
each individual state).

This derives a SHA-256 hash from the canonical package name (well, not
canonical acccording to TAMER, but close: without the leading slash),
truncating it to 32 bits.  I used a birthday attack to estimate what the
size of this value ought to be: sqrt(2^32) = 65536, which is way more
packages than the poor XSLT-based compiler is going to handle.

If ever it needs to be increased due to conflicts, that is simple enough.

DEV-14965

src/current/include/dslc-base.xsl

@@ -26,7 +26,9 @@
   template that is intended for use with dslc should include this.
 -->
 <stylesheet version="2.0"
-            xmlns="http://www.w3.org/1999/XSL/Transform">
+            xmlns="http://www.w3.org/1999/XSL/Transform"
+            xmlns:xs="http://www.w3.org/2001/XMLSchema"
+            xmlns:preproc="http://www.lovullo.com/rater/preproc">
 
 <!--
   Absolute path to root of TAME
@@ -80,6 +82,26 @@
 -->
 <param name="__rseed" />
 
+<!--
+  A package-unique string
+
+  You should use `preproc:pkg-generate-id` instead of this value directly.
+
+  This value is deterministic, derived from `__srcpkg`, and so will not
+  change between runs; it can be used to generate identifier names that are
+  unique across packages, which is not something that we can rely on
+  `generate-id()` for on its own.
+
+  In practice, this can be concatenated with other generated strings,
+  including `generate-id()`-derived strings.
+
+  _There is no guarantee that this string will begin with a letter_, so you
+  should generate your identifiers accordingly.
+
+  See `DslCompiler.java` for implementation.
+-->
+<param name="__pkguniq" as="xs:string" />
+
 
 <!--
   Root node of template on which stylesheet was invoked
@@ -114,4 +136,12 @@
   </choose>
 </template>
 
+<function name="preproc:pkg-generate-id" as="xs:string">
+  <param name="refnode" as="node()" />
+
+  <sequence select="concat(
+                      '_pu', $__pkguniq, '_',
+                      generate-id( $refnode ) )" />
+</function>
+
 </stylesheet>

src/current/include/preproc/expand.xsl

@@ -166,7 +166,7 @@
 <template match="c:let[ not( @name ) ]" mode="preproc:expand" priority="5">
   <copy>
     <sequence select="@*" />
-    <attribute name="name" select="generate-id(.)" />
+    <attribute name="name" select="preproc:pkg-generate-id(.)" />
 
     <apply-templates select="*" mode="preproc:expand" />
   </copy>

src/current/include/preproc/macros.xsl

@@ -415,11 +415,7 @@
 <template match="lv:any|lv:all" mode="preproc:class-groupgen" priority="5">
   <param name="legacy-classify" as="xs:boolean" tunnel="yes" />
 
-  <!-- this needs to be unique enough that there is unlikely to be a conflict
+  <variable name="id" select="preproc:pkg-generate-id(.)" />
-       between generated ids in various packages; generate-id is not enough for
-       cross-package guarantees (indeed, I did witness conflicts), so there is
-       a random seed passed into the stylesheet externally -->
-  <variable name="id" select="concat( $__rseed, generate-id(.) )" />
 
   <variable name="parent-name" select="ancestor::lv:classify/@as" />
   <variable name="yields" select="concat( 'is', $id )" />

src/current/include/preproc/template.xsl

@@ -411,7 +411,7 @@
   with an optional looping construct
 -->
 <template match="lv:inline-template" mode="preproc:macros" priority="5">
-  <variable name="name" select="concat( '___i', generate-id(.), '___' )" />
+  <variable name="name" select="concat( '___i', preproc:pkg-generate-id(.), '___' )" />
   <variable name="inline" select="." />
 
   <!-- generate template -->
@@ -1155,7 +1155,7 @@
              tunnel="yes" />
 
   <value-of select="." />
-  <value-of select="generate-id( $apply )" />
+  <value-of select="preproc:pkg-generate-id( $apply )" />
 </template>
 
 

src/current/src/com/lovullo/dslc/DslCompiler.java

@@ -31,6 +31,7 @@ package com.lovullo.dslc;
 import java.io.*;
 import java.nio.file.Path;
 import java.nio.file.Paths;
+import java.security.MessageDigest;
 import java.util.HashMap;
 import java.util.Map;
 import javax.xml.transform.Source;
@@ -163,6 +164,10 @@ public class DslCompiler
                 new QName( "__rseed" ),
                 XdmValue.makeValue( (int)( Math.random() * 10e6 ) )
             );
+            t.setParameter(
+                new QName( "__pkguniq" ),
+                XdmValue.makeValue( _createPkgUniq( srcpkg ) )
+            );
 
             _setTemplateParams( t, params );
 
@@ -173,6 +178,51 @@ public class DslCompiler
         }
 
 
+        // Generate an identifier that is expected to be unique given a
+        // canonical package name.
+        //
+        // This produces a string that is expected to
+        //   - Be usable as a non-leading component of a C-style identifier;
+        //   - Provide enough entropy so as to be unlikely to cause
+        //     conflicts between thousands of packages; and
+        //   - Is reasonably short so as not to bloat generated identifier
+        //     sizes too greatly.
+        private String _createPkgUniq(String srcpkg) throws Exception {
+            // This is used only for uniqueness, not security.  The choice
+            // of hash function is not particularly important so long as it
+            // provides a good distribution.  If collisions are encountered
+            // between packages with a good algorithm, increase the
+            // truncation length.
+            MessageDigest md = MessageDigest.getInstance( "SHA-256" );
+            byte[] digest = md.digest( srcpkg.getBytes( "UTF-8" ) );
+
+            // Ensure we received a string of the expected length, otherwise
+            // we risk sending a non-unique id off to the XSLT-based
+            // compiler.
+            int len_expected = 32; // 256 bits
+            int len = digest.length;
+            if ( len != len_expected ) {
+                throw new Exception(
+                    String.format(
+                        "Unexpected pkguniq length (expected %d): %d",
+                        len_expected,
+                        len
+                    )
+                );
+            }
+
+            // 32 bits = 4 bytes = 8 hex chars.
+            // Birthday attack: sqrt(2^32) = 65536 packages
+            StringBuilder hex = new StringBuilder();
+            int use_len = 4;
+            for ( int i = 0; i < use_len; i++ ) {
+                hex.append( String.format( "%02x", digest[i] ) );
+            }
+
+            return hex.toString();
+        }
+
+
         private void _setTemplateParams(
             XsltTransformer t,
             HashMap<String,String> params