build_static_site.sh: create target if nonexistent (#695)

Summary:
The current version of the build script has the safe but annoying
property that the target directory must be an existing, empty directory.
It seems reasonable and convenient to allow the build script to create
the directory with `mkdir -p`. It still fails if the directory is not
empty or is a file.

Test Plan:
Unit tests updated; run `yarn sharness-full`.

wchargin-branch: build-mkdir-p

scripts/build_static_site.sh

@@ -45,9 +45,7 @@ parse_args() {
                 fi
                 shift
                 if [ $# -eq 0 ]; then die 'missing value for --target'; fi
-                if ! target="$(readlink -e "$1")"; then
-                    die "target does not exist: $1"
-                fi
+                target="$1"
                 ;;
             --repo)
                 shift
@@ -79,12 +77,16 @@ parse_args() {
     if [ -z "${target}" ]; then
         die 'target directory not specified'
     fi
+    if ! [ -e "${target}" ]; then
+        mkdir -p -- "${target}"
+    fi
     if ! [ -d "${target}" ]; then
         die "target is not a directory: ${target}"
     fi
     if [ "$(command ls -A "${target}" | wc -l)" != 0 ]; then
         die "target directory is nonempty: ${target}"
     fi
+    target="$(readlink -e "${target}")"
 }
 
 build() {

sharness/test_build_static_site.t

@@ -48,21 +48,17 @@ test_expect_success "should fail with multiple targets" '
     grep -qF -- "--target specified multiple times" err
 '
 
-test_expect_success "should fail with nonexistent targets" '
-    test_must_fail run --target wat 2>err &&
-    grep -qF -- "target does not exist: wat" err
-'
-
-test_expect_success "should fail with nonexistent targets with subcomponents" '
-    # using "readlink -f", this behavior can be different.
-    test_must_fail run --target wat/wat 2>err &&
-    grep -qF -- "target does not exist: wat/wat" err
-'
-
 test_expect_success "should fail with a file as target" '
     printf "important\nstuff" >important_data &&
     test_must_fail run --target important_data 2>err &&
-    grep -qF -- "target is not a directory: ${PWD}/important_data" err &&
+    grep -qF -- "target is not a directory" err &&
+    printf "important\nstuff" | test_cmp - important_data
+'
+
+test_expect_success "should fail with a target under a file" '
+    printf "important\nstuff" >important_data &&
+    test_must_fail run --target important_data/something 2>err &&
+    grep -q -- "cannot create directory.*Not a directory" err &&
     printf "important\nstuff" | test_cmp - important_data
 '
 
@@ -70,7 +66,7 @@ test_expect_success "should fail with a nonempty directory as target" '
     mkdir important_dir &&
     printf "redacted\n" >important_dir/.wallet.dat &&
     test_must_fail run --target important_dir 2>err &&
-    grep -qF -- "target directory is nonempty: ${PWD}/important_dir" err &&
+    grep -qF -- "target directory is nonempty: important_dir" err &&
     printf "redacted\n" | test_cmp - important_dir/.wallet.dat
 '
 
@@ -116,7 +112,7 @@ fi
 run_build() {
     prereq_name="$1"; shift
     description="$1"; shift
-    output_dir="output_${prereq_name}"
+    output_dir="build_output/output_${prereq_name}"
     api_dir="${output_dir}/api/v1/data"
     data_dir="${api_dir}/data"
     for arg in "${output_dir}" "$@"; do
@@ -129,7 +125,6 @@ run_build() {
     flags="--target $output_dir $*"  # checked for sanity above
     test_expect_success EXPENSIVE,HAVE_GITHUB_TOKEN \
         "${prereq_name}: ${description}" '
-        mkdir "${output_dir}" &&
         run '"${flags}"' 2>err &&
         test_must_fail grep -vF \
             -e "Removing build directory: " \