Make add-dependency idempotent (#8534)

### Motivation:

If the same dependency is added twice with different version specifiers
the manifest is no longer valid. If the same dependency is added twice
with the same version specifier it remains valid, but less maintainable.

### Modifications:

If a manifest already contains the exact dependency and version that is
being added via a `add-dependency` call, don't add it twice. Instead, do
nothing.

If a manifest contains a dependency with the supplied URL but a
different version qualifier then throw an error explaining that the
supplied dependency has already been added.

### Result:

A manifest that continues to parse correctly.

Issue: #8519

Author: plemarquand

Sources/PackageModelSyntax/AddPackageDependency.swift

@@ -44,6 +44,13 @@ public enum AddPackageDependency {
             throw ManifestEditError.cannotFindPackage
         }
 
+        guard try !dependencyAlreadyAdded(
+            dependency,
+            in: packageCall
+        ) else {
+            return PackageEditResult(manifestEdits: [])
+        }
+
         let newPackageCall = try addPackageDependencyLocal(
             dependency, to: packageCall
         )
@@ -55,6 +62,50 @@ public enum AddPackageDependency {
         )
     }
 
+    /// Return `true` if the dependency already exists in the manifest, otherwise return `false`.
+    /// Throws an error if a dependency already exists with the same id or url, but different arguments.
+    private static func dependencyAlreadyAdded(
+        _ dependency: MappablePackageDependency.Kind,
+        in packageCall: FunctionCallExprSyntax
+    ) throws -> Bool {
+        let dependencySyntax = dependency.asSyntax()
+        guard let dependenctFnSyntax = dependencySyntax.as(FunctionCallExprSyntax.self) else {
+            throw ManifestEditError.cannotFindPackage
+        }
+
+        guard let id = dependenctFnSyntax.arguments.first(where: {
+            $0.label?.text == "url" || $0.label?.text == "id" || $0.label?.text == "path"
+        }) else {
+            throw InternalError("Missing id or url argument in dependency syntax")
+        }
+
+        if let existingDependencies = packageCall.findArgument(labeled: "dependencies") {
+            // If we have an existing dependencies array, we need to check if
+            if let expr = existingDependencies.expression.as(ArrayExprSyntax.self) {
+                // Iterate through existing dependencies and look for an argument that matches
+                // either the `id` or `url` argument of the new dependency. 
+                let existingArgument = expr.elements.first { elem in
+                    if let funcExpr = elem.expression.as(FunctionCallExprSyntax.self) {
+                        return funcExpr.arguments.contains {
+                            $0.trimmedDescription == id.trimmedDescription
+                        }
+                    }
+                    return true
+                }
+
+                if let existingArgument {
+                    let normalizedExistingArgument = existingArgument.detached.with(\.trailingComma, nil)
+                    // This exact dependency already exists, return false to indicate we should do nothing.
+                    if normalizedExistingArgument.trimmedDescription == dependencySyntax.trimmedDescription {
+                        return true
+                    }
+                    throw ManifestEditError.existingDependency(dependencyName: dependency.identifier)
+                }
+            }
+        }
+        return false
+    }
+
     /// Implementation of adding a package dependency to an existing call.
     static func addPackageDependencyLocal(
         _ dependency: MappablePackageDependency.Kind,
@@ -67,3 +118,16 @@ public enum AddPackageDependency {
         )
     }
 }
+
+fileprivate extension MappablePackageDependency.Kind {
+    var identifier: String {
+        switch self {
+            case .sourceControl(let name, let path, _):
+                return name ?? path
+            case .fileSystem(let name, let location):
+                return name ?? location
+            case .registry(let id, _):
+                return id
+        }
+    }
+}

Sources/PackageModelSyntax/ManifestEditError.swift

@@ -23,6 +23,7 @@ package enum ManifestEditError: Error {
     case cannotFindArrayLiteralArgument(argumentName: String, node: Syntax)
     case oldManifest(ToolsVersion, expected: ToolsVersion)
     case cannotAddSettingsToPluginTarget
+    case existingDependency(dependencyName: String)
 }
 
 extension ToolsVersion {
@@ -46,6 +47,8 @@ extension ManifestEditError: CustomStringConvertible {
             "package manifest version \(version) is too old: please update to manifest version \(expectedVersion) or newer"
         case .cannotAddSettingsToPluginTarget:
             "plugin targets do not support settings"
+        case .existingDependency(let name):
+            "unable to add dependency '\(name)' because it already exists in the list of dependencies"
         }
     }
 }