fix: ensure backward compat w/ test

docs/pages/docs/providers/go.md

@@ -34,12 +34,12 @@ go get
 ## Build
 
 If your project has multiple binaries, you can specify which one to run with the `NIXPACKS_GO_BIN` environment variable.
-Otherwise, the first binary found in the project's `cmd` directory the in the project's root directory will be used.
+Otherwise, the first binary found in the project's root directory or the project's `cmd` directory will be used.
 
 ```
-go build -o out ./cmd/{name}
-# Or
 go build -o out
+# Or if there are no .go files in the root directory
+go build -o out ./cmd/{name}
 
 ```
 

examples/go-gin/cmd/server/main.go

@@ -0,0 +1,17 @@
+package main
+
+import (
+	"github.com/gin-gonic/gin"
+)
+
+var Router *gin.Engine
+
+func main() {
+	r := gin.Default()
+	r.GET("/", func(c *gin.Context) {
+		c.JSON(200, gin.H{
+			"message": "Hello world!",
+		})
+	})
+	r.Run()
+}

src/providers/go.rs

@@ -65,26 +65,38 @@ impl Provider for GolangProvider {
         setup.set_nix_archive(archive);
 
         plan.add_phase(setup);
+        let is_go_module = app.includes_file("go.mod");
 
-        if app.includes_file("go.mod") {
+        if is_go_module {
             let mut install = Phase::install(Some("go mod download".to_string()));
             install.add_cache_directory(GO_BUILD_CACHE_DIR.to_string());
             plan.add_phase(install);
         }
 
+        let has_root_go_files = app.find_files("*.go").ok().map_or(false, |files| {
+            files
+                .iter()
+                .any(|file| file.parent() == Some(app.source.as_path()))
+        });
+
         let build_command = if let Some(name) = env.get_config_variable("GO_BIN") {
             Some(format!("go build -o {BINARY_NAME} ./cmd/{name}"))
+        } else if is_go_module && has_root_go_files {
+            Some(format!("go build -o {BINARY_NAME}"))
         } else if app.includes_directory("cmd") {
             // Try to find a command in the cmd directory
             app.find_directories("cmd/*")
                 .ok()
-                .and_then(|dirs| dirs.into_iter().next())
+                .and_then(|dirs| {
+                    dirs.into_iter()
+                        .find(|path| path.parent().map_or(false, |p| p.ends_with("cmd")))
+                })
                 .and_then(|path| {
                     path.file_name()
                         .and_then(|os_str| os_str.to_str())
                         .map(|name| format!("go build -o {BINARY_NAME} ./cmd/{name}"))
                 })
-        } else if app.includes_file("go.mod") {
+        } else if is_go_module {
             Some(format!("go build -o {BINARY_NAME}"))
         } else if app.includes_file("main.go") {
             Some(format!("go build -o {BINARY_NAME} main.go"))