Enable caching/incremental builds for Windows builds on travis.

Problem: msbuild uses timestamps to determine what needs to be rebuilt, and git doesn't track files' last modified time, so everything is always fully rebuilt.
Solution: This commit adds an sqlite database that tracks C/C++ files and their md5 hashes. If a file's hash hasn't changed, then it doesn't need to be rebuilt, so its last modified time is backdated 20 years. The 20 years is an arbitrarily chosen amount of time and has no special significance.

.travis.yml

@@ -9,6 +9,8 @@ cache:
   directories:
     - $HOME/build-cache
     - $HOME/flatpak-cache
+    - $TRAVIS_BUILD_DIR/projectfiles/VC14/Debug
+    - $TRAVIS_BUILD_DIR/projectfiles/VC14/Release
 
 git:
   depth: 5

utils/travis/steps/install.sh

@@ -28,6 +28,7 @@ if [ "$TRAVIS_OS_NAME" = "osx" ]; then
     fi
 elif [ "$TRAVIS_OS_NAME" = "windows" ]; then
     start=`pwd`
+    choco install sqlite
     choco install python --version=3.6.8
     cd /c/Python36
     ln -s python.exe python3.exe
@@ -38,12 +39,14 @@ elif [ "$TRAVIS_OS_NAME" = "windows" ]; then
     mv external-VC15 external
     cd $start
     export PATH="/c/Python36:"$PATH":/c/Program Files (x86)/Microsoft Visual Studio/2017/BuildTools/MSBuild/15.0/Bin/amd64:$start/../external/dll"
-    if [ "$(which python3)" == "" ] || [ ! -d "../external" ]; then
+    if [ "$(which python3)" == "" ] || [ "$(which sqlite3)" == "" ] || [ ! -d "../external" ]; then
       echo "Failed to retrieve dependencies!"
       exit 1
     else
       echo "Dependencies retrieved and installed!"
     fi
+
+    ./utils/travis/windows-file-hasher.sh "projectfiles/VC14/$OPT/filehashes.sqlite"
 else
     if [ "$NLS" != "true" ]; then
         echo "po/" >> .dockerignore

utils/travis/steps/script.sh

@@ -42,11 +42,19 @@ if [ "$TRAVIS_OS_NAME" = "osx" ]; then
 elif [ "$TRAVIS_OS_NAME" = "windows" ]; then
     powershell "MSBuild.exe projectfiles/VC14/wesnoth.sln -p:PlatformToolset=v141 -p:Configuration=$OPT"
     BUILD_RET=$?
+
+    if [ "$BUILD_RET" != "0" ]; then
+        sqlite3 "projectfiles/VC14/$OPT/filehashes.sqlite" "update FILES set MD5 = OLD_MD5, OLD_MD5 = '-' where OLD_MD5 != '-'"
+    else
+        sqlite3 "projectfiles/VC14/$OPT/filehashes.sqlite" "update FILES set OLD_MD5 = '-' where OLD_MD5 != '-'"
+    fi
+
     if [ "$OPT" == "Release" ] && [ "$BUILD_RET" == "0" ]; then
         ./run_wml_tests -g -v -c -t "$WML_TEST_TIME"
-    else
-        exit $BUILD_RET
+        BUILD_RET=$?
     fi
+
+    exit $BUILD_RET
 else
 # additional permissions required due to flatpak's use of bubblewrap
     docker run --cap-add=ALL --privileged \

utils/travis/windows-file-hasher.sh

@@ -0,0 +1,32 @@
+#!/bin/bash
+DBFILE="$1"
+if [ "$DBFILE" == "" ]; then
+  echo "No database file provided!"
+  exit 1
+fi
+
+sqlite3 $DBFILE "create table if not exists FILES(NAME VARCHAR(1000) PRIMARY KEY, MD5 VARCHAR(32), OLD_MD5 VARCHAR(32) DEFAULT '-')"
+
+for file in $(find . -type f | grep '\.c$\|\.cpp$\|\.h$\|\.hpp$\|\.ipp$\|\.tpp$\|\.ii' | cut -c 3-); do
+  newhash=`md5sum "$file" | cut -d" " -f1`
+  oldhash=`sqlite3 "$DBFILE" "select MD5 from FILES where NAME = '"$file"'"`
+
+  if [ "$oldhash" == "" ]; then
+    sqlite3 "$DBFILE" "insert into FILES(NAME, MD5) values('$file', '$newhash')"
+    printf '%-15s %-60s has %-32s\n' "new file:" "$file" "$newhash"
+  elif [ "$oldhash" != "$newhash" ]; then
+    sqlite3 "$DBFILE" "update FILES set OLD_MD5 = MD5, MD5 = '$newhash' where NAME = '$file'"
+    printf '%-15s %-60s had %-32s %-13s %-32s\n' "changed file:" "$file" "$oldhash" "and now has" "$newhash"
+  else
+    printf '%-15s %-60s had %-32s %-13s %-32s\n' "unchanged file:" "$file" "$oldhash" "and still has" "$newhash"
+    touch -d "20 years ago" "$file"
+  fi
+done
+
+for file in `sqlite3 "$DBFILE" "select NAME from FILES"`; do
+  file=`echo "$file" | tr -d '\r'`
+  if [ ! -f "$file" ]; then
+    printf '%-15s %-60s\n' "File removed:" "$file"
+    sqlite3 "$DBFILE" "delete from FILES where NAME = '$file'"
+  fi
+done