Reimplement most of the tool:

Most of the tool has been reimplemented, with the exception of the
permission editing feature, which is currently half implemented. There
are also several TODOs spread around that would benefit from some action

src/core/user_operations.rs

@@ -0,0 +1,151 @@
+use anyhow::Context;
+use indoc::indoc;
+use nix::unistd::User;
+use serde::{Deserialize, Serialize};
+use sqlx::{prelude::*, MySqlConnection};
+
+use crate::core::common::quote_literal;
+
+use super::common::{get_current_unix_user, get_unix_groups, validate_prefix_for_user};
+
+pub async fn create_database_user(db_user: &str, conn: &mut MySqlConnection) -> anyhow::Result<()> {
+    let unix_user = get_current_unix_user()?;
+
+    validate_ownership_of_user_name(db_user, &unix_user)?;
+
+    // NOTE: see the note about SQL injections in `validate_ownershipt_of_user_name`
+    sqlx::query(format!("CREATE USER {}@'%'", quote_literal(db_user),).as_str())
+        .execute(conn)
+        .await?;
+
+    Ok(())
+}
+
+pub async fn delete_database_user(db_user: &str, conn: &mut MySqlConnection) -> anyhow::Result<()> {
+    let unix_user = get_current_unix_user()?;
+
+    validate_ownership_of_user_name(db_user, &unix_user)?;
+
+    // NOTE: see the note about SQL injections in `validate_ownershipt_of_user_name`
+    sqlx::query(format!("DROP USER {}@'%'", quote_literal(db_user),).as_str())
+        .execute(conn)
+        .await?;
+
+    Ok(())
+}
+
+pub async fn set_password_for_database_user(
+    db_user: &str,
+    password: &str,
+    conn: &mut MySqlConnection,
+) -> anyhow::Result<()> {
+    let unix_user = crate::core::common::get_current_unix_user()?;
+    validate_ownership_of_user_name(db_user, &unix_user)?;
+
+    // NOTE: see the note about SQL injections in `validate_ownershipt_of_user_name`
+    sqlx::query(
+        format!(
+            "ALTER USER {}@'%' IDENTIFIED BY {}",
+            quote_literal(db_user),
+            quote_literal(password).as_str()
+        )
+        .as_str(),
+    )
+    .execute(conn)
+    .await?;
+
+    Ok(())
+}
+
+#[derive(Debug, Clone, FromRow, Serialize, Deserialize)]
+pub struct DatabaseUser {
+    #[sqlx(rename = "User")]
+    pub user: String,
+
+    #[sqlx(rename = "Host")]
+    pub host: String,
+
+    #[sqlx(rename = "Password")]
+    pub password: String,
+
+    pub authentication_string: String,
+}
+
+pub async fn get_all_database_users_for_user(
+    user: &User,
+    conn: &mut MySqlConnection,
+) -> anyhow::Result<Vec<DatabaseUser>> {
+    let groups = get_unix_groups(user)?;
+
+    let regex = format!(
+        "({}|{})(_.+)?",
+        user.name,
+        groups
+            .iter()
+            .map(|g| g.name.as_str())
+            .collect::<Vec<_>>()
+            .join("|")
+    );
+
+    let users = sqlx::query_as::<_, DatabaseUser>(
+        r#"
+          SELECT `User`, `Host`, `Password`, `authentication_string`
+          FROM `mysql`.`user`
+          WHERE `User` REGEXP ?
+        "#,
+    )
+    .bind(regex)
+    .fetch_all(conn)
+    .await?;
+
+    Ok(users)
+}
+
+pub async fn get_database_user_for_user(
+    username: &str,
+    conn: &mut MySqlConnection,
+) -> anyhow::Result<Option<DatabaseUser>> {
+    let user = sqlx::query_as::<_, DatabaseUser>(
+        r#"
+          SELECT `User`, `Host`, `Password`, `authentication_string`
+          FROM `mysql`.`user`
+          WHERE `User` = ?
+        "#,
+    )
+    .bind(username)
+    .fetch_optional(conn)
+    .await?;
+
+    Ok(user)
+}
+
+/// NOTE: It is very critical that this function validates the database name
+///       properly. MySQL does not seem to allow for prepared statements, binding
+///       the database name as a parameter to the query. This means that we have
+///       to validate the database name ourselves to prevent SQL injection.
+pub fn validate_ownership_of_user_name(name: &str, user: &User) -> anyhow::Result<()> {
+    if name.contains(|c: char| !c.is_ascii_alphanumeric() && c != '_' && c != '-') {
+        anyhow::bail!(
+            indoc! {r#"
+              Username '{}' contains invalid characters.
+              Only A-Z, a-z, 0-9, _ (underscore) and - (dash) permitted.
+            "#},
+            name
+        );
+    }
+
+    // TODO: does the name have a length limit?
+    // if name.len() > 48 {
+    //     anyhow::bail!(
+    //         indoc! {r#"
+    //           Username '{}' is too long.
+    //           Maximum length is 48 characters. Skipping.
+    //         "#},
+    //         name
+    //     );
+    // }
+
+    validate_prefix_for_user(name, user).context(format!("Invalid username: '{}'", name))?;
+
+    Ok(())
+}